#include <bits/stdc++.h>
using namespace std;
int n, c, k, arr[500], res;
int main() {
  cin >> n >> k >> c;
  for (int i = 1; i <= c; i++) {
    int a;
    cin >> a;
    arr[a] = 1;
  }
  c = 1;
  for (int i = 1; i <= n; i++, c++) {
    if (arr[i] == 1 || c == k) {
      c = 0;
      res++;
    }
  }
  cout << res;
  return 0;
}
