diff --git a/goTool/README.md b/goTool/README.md
index 687f2c3..2a78439 100644
--- a/goTool/README.md
+++ b/goTool/README.md
@@ -31,7 +31,7 @@ Example output:
 
 ```
 clients (2):
-  [0] id=42 mac=aabbccddeeff ver=1 available=true used=false last_ping=12345 last_success_ping=12345
+  [0] id=42 mac=aabbccddeeff ver=1 available=true used=false last_ping=250 last_success_ping=250
 ```
 
 ## Regenerate protobuf
diff --git a/main/README.md b/main/README.md
index bf0c194..eb4a295 100644
--- a/main/README.md
+++ b/main/README.md
@@ -77,6 +77,7 @@ WiFi is brought up in STA mode (no AP association). Channel = `app_config.networ
 |------|-------|-----------|---------|
 | `DISCOVER` | 1 | Master → broadcast `FF:FF:FF:FF:FF:FF` | Master is searching for slaves |
 | `SLAVE_INFO` | 2 | Slave → master | Slave registration |
+| `HEARTBEAT` | 3 | Slave → master | Keep-alive (header only, no extra payload) |
 
 `SLAVE_INFO` payload (after header bytes 0–2):
 
@@ -88,9 +89,9 @@ WiFi is brought up in STA mode (no AP association). Channel = `app_config.networ
 | `available` | uint8 | 1 = available |
 | `used` | uint8 | 0 = unused |
 
-**Master:** task `espnow_disc` sends `DISCOVER` every **500 ms** on the configured network. Logs `slave joined id=… mac=… ver=…` when a new slave is seen (up to 16 entries).
+**Master:** task `espnow_disc` sends `DISCOVER` every **500 ms** on the configured network. Logs `slave joined id=… mac=… ver=…` when a new slave is seen (up to 16 entries). Task `espnow_mon` runs every **1 s** and marks a client **inactive** (`available = false`) if no `SLAVE_INFO` or `HEARTBEAT` was received for **3 s** (three missed 1 s heartbeats). A later heartbeat sets `available` true again and logs reactivation.
 
-**Slave:** on first matching `DISCOVER`, logs `joined network N, master …`, sends `SLAVE_INFO` once, then ignores further discovers from that master (no repeat log or reply).
+**Slave:** on first matching `DISCOVER`, logs `joined network N, master …`, sends `SLAVE_INFO` once, then sends `HEARTBEAT` to the master every **1 s**. While joined, periodic discovers from the same master refresh a “master alive” timer; if no discover arrives for **5 s**, the slave treats the master as lost, clears join state, and will register again on the next discover (reconnect). Discover from a different master is ignored while already joined.
 
 Monitor via USB-JTAG (`/dev/ttyACM0`) while using a USB-serial adapter on **GPIO2/3** (`/dev/ttyUSB0`) for UART — they are different interfaces.
 
@@ -188,7 +189,7 @@ Encoding: `uart_send_uart_message()` in `uart_proto.c`.
 
 **Response:** payload `04` + nanopb `UartMessage` with `client_info_response.clients` — one `ClientInfo` per registered slave (from ESP-NOW `SLAVE_INFO`).
 
-Fields per client: `id`, `mac`, `version`, `available`, `used`, `last_ping`, `last_success_ping` (milliseconds since boot, updated on each `SLAVE_INFO`).
+Fields per client: `id`, `mac`, `version`, `available`, `used`, `last_ping`, `last_success_ping` — **milliseconds since** the last packet / last successful heartbeat (computed when `CLIENT_INFO` is answered; typically 0–1000 while the slave is heartbeating every 1 s).
 
 ## Client registry
 
@@ -196,9 +197,11 @@ Fields per client: `id`, `mac`, `version`, `available`, `used`, `last_ping`, `la
 |-----|-------------|
 | `client_registry_init()` | Clear all slots (called from `esp_now_comm_init`) |
 | `client_registry_upsert(mac, id, version, …)` | Add or refresh client; updates ping timestamps |
+| `client_registry_heartbeat(mac, id, version, …)` | Same as upsert for heartbeats; reactivates inactive clients |
+| `client_registry_check_timeouts(timeout_ms)` | Mark stale clients inactive (master monitor task) |
 | `client_registry_count()` / `client_registry_at(i)` | Iterate for UART encoding |
 
-Slaves register when the master receives `SLAVE_INFO` on the matching network.
+Slaves register when the master receives `SLAVE_INFO` on the matching network; `HEARTBEAT` keeps them marked available.
 
 ## Host tool (`goTool/`)
 
diff --git a/main/client_registry.c b/main/client_registry.c
index 03dc969..4514015 100644
--- a/main/client_registry.c
+++ b/main/client_registry.c
@@ -1,8 +1,12 @@
 #include "client_registry.h"
+#include "esp_log.h"
 #include "freertos/FreeRTOS.h"
 #include "freertos/idf_additions.h"
+#include <stdio.h>
 #include <string.h>
 
+static const char *TAG = "[CLIENTS]";
+
 typedef struct {
   client_info_t info;
   bool active;
@@ -10,27 +14,67 @@ typedef struct {
 
 static client_slot_t s_clients[CLIENT_REGISTRY_MAX];
 
-static uint32_t now_ms(void) {
+uint32_t client_registry_now_ms(void) {
   return (uint32_t)(xTaskGetTickCount() * portTICK_PERIOD_MS);
 }
 
+uint32_t client_registry_ms_since(uint32_t timestamp) {
+  if (timestamp == 0) {
+    return 0;
+  }
+  return client_registry_now_ms() - timestamp;
+}
+
+static uint32_t now_ms(void) { return client_registry_now_ms(); }
+
 static bool mac_equal(const uint8_t *a, const uint8_t *b) {
   return memcmp(a, b, CLIENT_MAC_LEN) == 0;
 }
 
+static void mac_to_str(const uint8_t *mac, char *out, size_t out_len) {
+  snprintf(out, out_len, "%02x:%02x:%02x:%02x:%02x:%02x", mac[0], mac[1],
+           mac[2], mac[3], mac[4], mac[5]);
+}
+
+static client_slot_t *find_slot(const uint8_t mac[CLIENT_MAC_LEN]) {
+  for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
+    if (s_clients[i].active && mac_equal(s_clients[i].info.mac, mac)) {
+      return &s_clients[i];
+    }
+  }
+  return NULL;
+}
+
+static client_slot_t *alloc_slot(const uint8_t mac[CLIENT_MAC_LEN],
+                                 bool *out_is_new) {
+  client_slot_t *slot = find_slot(mac);
+  if (slot != NULL) {
+    if (out_is_new != NULL) {
+      *out_is_new = false;
+    }
+    return slot;
+  }
+
+  for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
+    if (!s_clients[i].active) {
+      slot = &s_clients[i];
+      slot->active = true;
+      memcpy(slot->info.mac, mac, CLIENT_MAC_LEN);
+      if (out_is_new != NULL) {
+        *out_is_new = true;
+      }
+      return slot;
+    }
+  }
+  return NULL;
+}
+
 void client_registry_init(void) { memset(s_clients, 0, sizeof(s_clients)); }
 
 const client_info_t *
 client_registry_find_by_mac(const uint8_t mac[CLIENT_MAC_LEN]) {
-  if (mac == NULL) {
-    return NULL;
-  }
-  for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
-    if (s_clients[i].active && mac_equal(s_clients[i].info.mac, mac)) {
-      return &s_clients[i].info;
-    }
-  }
-  return NULL;
+  client_slot_t *slot = find_slot(mac);
+  return slot != NULL ? &slot->info : NULL;
 }
 
 esp_err_t client_registry_upsert(const uint8_t mac[CLIENT_MAC_LEN], uint32_t id,
@@ -41,37 +85,18 @@ esp_err_t client_registry_upsert(const uint8_t mac[CLIENT_MAC_LEN], uint32_t id,
   }
 
   uint32_t ts = now_ms();
-  client_slot_t *slot = NULL;
   bool is_new = false;
-
-  for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
-    if (s_clients[i].active && mac_equal(s_clients[i].info.mac, mac)) {
-      slot = &s_clients[i];
-      break;
-    }
-  }
-
+  client_slot_t *slot = alloc_slot(mac, &is_new);
   if (slot == NULL) {
-    for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
-      if (!s_clients[i].active) {
-        slot = &s_clients[i];
-        slot->active = true;
-        memcpy(slot->info.mac, mac, CLIENT_MAC_LEN);
-        is_new = true;
-        break;
-      }
-    }
-    if (slot == NULL) {
-      return ESP_ERR_NO_MEM;
-    }
+    return ESP_ERR_NO_MEM;
   }
 
   slot->info.id = id;
   slot->info.version = version;
   slot->info.available = available;
   slot->info.used = used;
-  slot->info.last_ping = ts;
-  slot->info.last_success_ping = ts;
+  slot->info.last_ping_at = ts;
+  slot->info.last_success_ping_at = ts;
 
   if (out_is_new != NULL) {
     *out_is_new = is_new;
@@ -79,6 +104,61 @@ esp_err_t client_registry_upsert(const uint8_t mac[CLIENT_MAC_LEN], uint32_t id,
   return ESP_OK;
 }
 
+esp_err_t client_registry_heartbeat(const uint8_t mac[CLIENT_MAC_LEN],
+                                    uint32_t id, uint32_t version, bool used,
+                                    bool *out_is_new, bool *out_reactivated) {
+  if (mac == NULL) {
+    return ESP_ERR_INVALID_ARG;
+  }
+
+  uint32_t ts = now_ms();
+  bool is_new = false;
+  bool reactivated = false;
+  client_slot_t *slot = alloc_slot(mac, &is_new);
+  if (slot == NULL) {
+    return ESP_ERR_NO_MEM;
+  }
+
+  if (!is_new && !slot->info.available) {
+    reactivated = true;
+  }
+
+  slot->info.id = id;
+  slot->info.version = version;
+  slot->info.used = used;
+  slot->info.available = true;
+  slot->info.last_ping_at = ts;
+  slot->info.last_success_ping_at = ts;
+
+  if (out_is_new != NULL) {
+    *out_is_new = is_new;
+  }
+  if (out_reactivated != NULL) {
+    *out_reactivated = reactivated || is_new;
+  }
+  return ESP_OK;
+}
+
+void client_registry_check_timeouts(uint32_t timeout_ms) {
+  uint32_t now = now_ms();
+
+  for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
+    if (!s_clients[i].active || !s_clients[i].info.available) {
+      continue;
+    }
+
+    uint32_t elapsed = now - s_clients[i].info.last_success_ping_at;
+    if (elapsed > timeout_ms) {
+      s_clients[i].info.available = false;
+      char mac_str[18];
+      mac_to_str(s_clients[i].info.mac, mac_str, sizeof(mac_str));
+      ESP_LOGW(TAG, "client inactive id=%lu mac=%s (no heartbeat for %lu ms)",
+               (unsigned long)s_clients[i].info.id, mac_str,
+               (unsigned long)elapsed);
+    }
+  }
+}
+
 size_t client_registry_count(void) {
   size_t n = 0;
   for (size_t i = 0; i < CLIENT_REGISTRY_MAX; i++) {
diff --git a/main/client_registry.h b/main/client_registry.h
index fc093b7..1ef6e8e 100644
--- a/main/client_registry.h
+++ b/main/client_registry.h
@@ -14,18 +14,38 @@ typedef struct {
   bool available;
   bool used;
   uint8_t mac[CLIENT_MAC_LEN];
-  uint32_t last_ping;
-  uint32_t last_success_ping;
+  /** Milliseconds since boot when last packet was received from this client. */
+  uint32_t last_ping_at;
+  /** Milliseconds since boot when last heartbeat / SLAVE_INFO was accepted. */
+  uint32_t last_success_ping_at;
   uint32_t version;
 } client_info_t;
 
 void client_registry_init(void);
 
-/** Register or refresh a client; sets last_ping and last_success_ping to now. */
+/** Milliseconds since boot (same clock as stored ping timestamps). */
+uint32_t client_registry_now_ms(void);
+
+/** Ms elapsed since timestamp; 0 if timestamp is 0. */
+uint32_t client_registry_ms_since(uint32_t timestamp);
+
+/** Register or refresh a client; updates both ping timestamps. */
 esp_err_t client_registry_upsert(const uint8_t mac[CLIENT_MAC_LEN], uint32_t id,
                                  uint32_t version, bool available, bool used,
                                  bool *out_is_new);
 
+/**
+ * Record a successful heartbeat (or initial slave info).
+ * Sets available=true and updates last_success_ping_at (and last_ping_at).
+ * If client was inactive, sets *out_reactivated=true.
+ */
+esp_err_t client_registry_heartbeat(const uint8_t mac[CLIENT_MAC_LEN],
+                                    uint32_t id, uint32_t version, bool used,
+                                    bool *out_is_new, bool *out_reactivated);
+
+/** Mark clients inactive when last_success_ping_at is older than timeout_ms. */
+void client_registry_check_timeouts(uint32_t timeout_ms);
+
 size_t client_registry_count(void);
 const client_info_t *client_registry_at(size_t index);
 const client_info_t *client_registry_find_by_mac(const uint8_t mac[CLIENT_MAC_LEN]);
diff --git a/main/cmd_client_info.c b/main/cmd_client_info.c
index 69c8a00..c59f57b 100644
--- a/main/cmd_client_info.c
+++ b/main/cmd_client_info.c
@@ -35,8 +35,9 @@ static bool encode_clients_list(pb_ostream_t *stream, const pb_field_t *field,
     proto.id = client->id;
     proto.available = client->available;
     proto.used = client->used;
-    proto.last_ping = client->last_ping;
-    proto.last_success_ping = client->last_success_ping;
+    proto.last_ping = client_registry_ms_since(client->last_ping_at);
+    proto.last_success_ping =
+        client_registry_ms_since(client->last_success_ping_at);
     proto.version = client->version;
     proto.mac.funcs.encode = encode_client_mac;
     proto.mac.arg = (void *)client->mac;
diff --git a/main/esp_now_comm.c b/main/esp_now_comm.c
index afd9469..37acc47 100644
--- a/main/esp_now_comm.c
+++ b/main/esp_now_comm.c
@@ -20,7 +20,14 @@
 #define ESPNOW_MAGIC 0xA1
 #define ESPNOW_MSG_DISCOVER 1
 #define ESPNOW_MSG_SLAVE_INFO 2
+#define ESPNOW_MSG_HEARTBEAT 3
 #define ESPNOW_DISCOVER_INTERVAL_MS 500
+#define ESPNOW_HEARTBEAT_INTERVAL_MS 1000
+#define ESPNOW_HEARTBEAT_MISS_COUNT 3
+#define ESPNOW_CLIENT_TIMEOUT_MS                                               \
+  (ESPNOW_HEARTBEAT_INTERVAL_MS * ESPNOW_HEARTBEAT_MISS_COUNT)
+#define SLAVE_MASTER_LOST_MS (ESPNOW_HEARTBEAT_INTERVAL_MS * 5)
+
 static const uint8_t ESPNOW_BCAST[ESP_NOW_ETH_ALEN] = {0xff, 0xff, 0xff,
                                                        0xff, 0xff, 0xff};
 
@@ -42,13 +49,18 @@ typedef struct __attribute__((packed)) {
   uint32_t slave_id;
   uint8_t available;
   uint8_t used;
-} espnow_slave_info_packet_t;
+} espnow_slave_packet_t;
 
 static app_config_t s_config;
 static uint8_t s_wifi_channel;
 static uint8_t s_own_mac[ESP_NOW_ETH_ALEN];
 static bool s_slave_joined;
 static uint8_t s_master_mac[ESP_NOW_ETH_ALEN];
+static uint32_t s_last_discover_ms;
+
+static uint32_t now_ms(void) {
+  return (uint32_t)(xTaskGetTickCount() * portTICK_PERIOD_MS);
+}
 
 static uint8_t network_to_channel(uint8_t network) {
   if (network < 1 || network > 13) {
@@ -86,58 +98,48 @@ static esp_err_t ensure_peer(const uint8_t *mac) {
 
 static esp_err_t ensure_broadcast_peer(void) { return ensure_peer(ESPNOW_BCAST); }
 
-static void send_slave_info(const uint8_t *dest_mac) {
-  espnow_slave_info_packet_t pkt = {
-      .magic = ESPNOW_MAGIC,
-      .type = ESPNOW_MSG_SLAVE_INFO,
-      .network = s_config.network,
-      .version = POWERPOD_FW_VERSION,
-      .slave_id = s_own_mac[5],
-      .available = 1,
-      .used = 0,
-  };
-  memcpy(pkt.mac, s_own_mac, ESP_NOW_ETH_ALEN);
+static void build_slave_packet(espnow_slave_packet_t *pkt, uint8_t type) {
+  pkt->magic = ESPNOW_MAGIC;
+  pkt->type = type;
+  pkt->network = s_config.network;
+  memcpy(pkt->mac, s_own_mac, ESP_NOW_ETH_ALEN);
+  pkt->version = POWERPOD_FW_VERSION;
+  pkt->slave_id = s_own_mac[5];
+  pkt->available = 1;
+  pkt->used = 0;
+}
+
+static void send_slave_packet(const uint8_t *dest_mac, uint8_t type) {
+  espnow_slave_packet_t pkt;
+  build_slave_packet(&pkt, type);
 
   if (ensure_peer(dest_mac) != ESP_OK) {
     return;
   }
 
-  esp_err_t err =
-      esp_now_send(dest_mac, (const uint8_t *)&pkt, sizeof(pkt));
+  esp_err_t err = esp_now_send(dest_mac, (const uint8_t *)&pkt, sizeof(pkt));
   if (err != ESP_OK) {
-    ESP_LOGW(TAG, "slave info send failed: %s", esp_err_to_name(err));
+    ESP_LOGW(TAG, "send type=%u failed: %s", (unsigned)type,
+             esp_err_to_name(err));
   }
 }
 
-static void handle_discover(const uint8_t *sender_mac,
-                            const espnow_discover_packet_t *pkt) {
-  if (pkt->network != s_config.network) {
-    return;
-  }
-
-  if (s_slave_joined && mac_equal(sender_mac, s_master_mac)) {
-    return;
-  }
-
-  memcpy(s_master_mac, sender_mac, ESP_NOW_ETH_ALEN);
-  s_slave_joined = true;
-
-  char mac_str[18];
-  mac_to_str(sender_mac, mac_str, sizeof(mac_str));
-  ESP_LOGI(TAG, "joined network %u, master %s", (unsigned)pkt->network, mac_str);
-
-  send_slave_info(sender_mac);
+static void slave_reset_join(void) {
+  s_slave_joined = false;
+  memset(s_master_mac, 0, sizeof(s_master_mac));
+  s_last_discover_ms = 0;
 }
 
-static void handle_slave_info(const espnow_slave_info_packet_t *pkt) {
+static void handle_client_packet(const espnow_slave_packet_t *pkt) {
   if (pkt->network != s_config.network) {
     return;
   }
 
   bool is_new = false;
-  esp_err_t err = client_registry_upsert(
-      pkt->mac, pkt->slave_id, pkt->version, pkt->available != 0,
-      pkt->used != 0, &is_new);
+  bool reactivated = false;
+  esp_err_t err = client_registry_heartbeat(
+      pkt->mac, pkt->slave_id, pkt->version, pkt->used != 0, &is_new,
+      &reactivated);
   if (err != ESP_OK) {
     ESP_LOGW(TAG, "client registry full");
     return;
@@ -149,6 +151,88 @@ static void handle_slave_info(const espnow_slave_info_packet_t *pkt) {
     ESP_LOGI(TAG, "client registered id=%lu mac=%s ver=%lu",
              (unsigned long)pkt->slave_id, mac_str,
              (unsigned long)pkt->version);
+  } else if (reactivated) {
+    ESP_LOGI(TAG, "client reconnected id=%lu mac=%s",
+             (unsigned long)pkt->slave_id, mac_str);
+  }
+}
+
+static void handle_discover(const uint8_t *sender_mac,
+                            const espnow_discover_packet_t *pkt) {
+  if (pkt->network != s_config.network) {
+    return;
+  }
+
+  uint32_t now = now_ms();
+
+  if (s_slave_joined) {
+    if (!mac_equal(sender_mac, s_master_mac)) {
+      return;
+    }
+    if ((now - s_last_discover_ms) <= SLAVE_MASTER_LOST_MS) {
+      s_last_discover_ms = now;
+      return;
+    }
+    ESP_LOGW(TAG, "master lost, rejoining");
+    slave_reset_join();
+  }
+
+  memcpy(s_master_mac, sender_mac, ESP_NOW_ETH_ALEN);
+  s_slave_joined = true;
+  s_last_discover_ms = now;
+
+  char mac_str[18];
+  mac_to_str(sender_mac, mac_str, sizeof(mac_str));
+  ESP_LOGI(TAG, "joined network %u, master %s", (unsigned)pkt->network, mac_str);
+
+  send_slave_packet(sender_mac, ESPNOW_MSG_SLAVE_INFO);
+}
+
+static void slave_check_master_timeout(void) {
+  if (!s_slave_joined) {
+    return;
+  }
+
+  uint32_t now = now_ms();
+  if (s_last_discover_ms == 0) {
+    return;
+  }
+
+  if ((now - s_last_discover_ms) > SLAVE_MASTER_LOST_MS) {
+    ESP_LOGW(TAG, "no master discover for %u ms, reconnecting",
+             (unsigned)(now - s_last_discover_ms));
+    slave_reset_join();
+  }
+}
+
+static void slave_heartbeat_task(void *param) {
+  (void)param;
+
+  ESP_LOGI(TAG, "slave heartbeat task (interval %u ms)",
+           (unsigned)ESPNOW_HEARTBEAT_INTERVAL_MS);
+
+  while (1) {
+    vTaskDelay(pdMS_TO_TICKS(ESPNOW_HEARTBEAT_INTERVAL_MS));
+
+    slave_check_master_timeout();
+
+    if (!s_slave_joined) {
+      continue;
+    }
+
+    send_slave_packet(s_master_mac, ESPNOW_MSG_HEARTBEAT);
+  }
+}
+
+static void master_monitor_task(void *param) {
+  (void)param;
+
+  ESP_LOGI(TAG, "master monitor task (timeout %u ms)",
+           (unsigned)ESPNOW_CLIENT_TIMEOUT_MS);
+
+  while (1) {
+    vTaskDelay(pdMS_TO_TICKS(ESPNOW_HEARTBEAT_INTERVAL_MS));
+    client_registry_check_timeouts(ESPNOW_CLIENT_TIMEOUT_MS);
   }
 }
 
@@ -170,8 +254,9 @@ static void espnow_recv_cb(const esp_now_recv_info_t *info, const uint8_t *data,
     }
     break;
   case ESPNOW_MSG_SLAVE_INFO:
-    if (s_config.master && len >= (int)sizeof(espnow_slave_info_packet_t)) {
-      handle_slave_info((const espnow_slave_info_packet_t *)data);
+  case ESPNOW_MSG_HEARTBEAT:
+    if (s_config.master && len >= (int)sizeof(espnow_slave_packet_t)) {
+      handle_client_packet((const espnow_slave_packet_t *)data);
     }
     break;
   default:
@@ -241,8 +326,7 @@ esp_err_t esp_now_comm_init(const app_config_t *config) {
   memset(&s_config, 0, sizeof(s_config));
   memcpy(&s_config, config, sizeof(s_config));
   client_registry_init();
-  s_slave_joined = false;
-  memset(s_master_mac, 0, sizeof(s_master_mac));
+  slave_reset_join();
 
   s_wifi_channel = network_to_channel(config->network);
   ESP_ERROR_CHECK(esp_read_mac(s_own_mac, ESP_MAC_WIFI_STA));
@@ -269,8 +353,17 @@ esp_err_t esp_now_comm_init(const app_config_t *config) {
       ESP_LOGE(TAG, "failed to create discover task");
       return ESP_FAIL;
     }
+    if (xTaskCreate(master_monitor_task, "espnow_mon", 4096, NULL, 4, NULL) !=
+        pdPASS) {
+      ESP_LOGE(TAG, "failed to create monitor task");
+      return ESP_FAIL;
+    }
   } else {
-    ESP_LOGI(TAG, "slave listening for master discover");
+    if (xTaskCreate(slave_heartbeat_task, "espnow_hb", 4096, NULL, 4, NULL) !=
+        pdPASS) {
+      ESP_LOGE(TAG, "failed to create heartbeat task");
+      return ESP_FAIL;
+    }
   }
 
   return ESP_OK;