#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c, a[1000], res = 0;
  cin >> n >> k >> c;
  for (int i = 0; i < c; i++) cin >> a[i];
  int lst = 0;
  int j = 0;
  for (int i = 1; i <= n; i++) {
    if (i == a[j] && j < c) {
      j++;
      lst = i;
      res++;
    } else if (i - lst == k) {
      lst = i;
      res++;
    }
  }
  cout << res;
  return 0;
}
