#include <bits/stdc++.h>
int main(void) {
  int n, k, c;
  static char days[366];
  int i;
  int count;
  int end;
  scanf("%d %d", &n, &k);
  memset(days, 0, n + 1);
  scanf("%d", &c);
  for (i = 0; i < c; i++) {
    int t;
    scanf("%d", &t);
    days[t] = 1;
  }
  end = k;
  for (i = 1, count = 0; i <= n; i++) {
    if (days[i] || i == end) {
      count++;
      end = i + k;
    }
  }
  printf("%d\n", count);
  return 0;
}
