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