#include <bits/stdc++.h>
using namespace std;
int n, k, used[1000], m;
int main() {
  scanf("%d%d", &n, &k);
  scanf("%d", &m);
  for (int i = 0; i < m; i++) {
    int x;
    scanf("%d", &x);
    used[x] = 1;
  }
  int next = k, p = 0, ans = 0;
  for (int i = 1; i <= n; i++) {
    if (i != next && used[i]) p = 0, ans++, next = i + k;
    if (i == next) {
      if (!p) ans++;
      next += k;
      p = 0;
    }
  }
  printf("%d", ans);
  return 0;
}
