#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, K, C;
  int days[366];
  int x;
  scanf("%d %d %d", &N, &K, &C);
  memset(days, 0, sizeof(days));
  for (int i = 0; i < C; i++) {
    scanf("%d", &x);
    days[x] = 1;
  }
  int lastPresent = 0;
  for (int i = 0; i <= N; i++) {
    if (days[i] == 1) lastPresent = i;
    if ((i - lastPresent) >= K) {
      days[i] = 1;
      lastPresent = i;
    }
  }
  printf("%d\n", accumulate(days, days + N + 1, 0));
}
