#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c, ans = 0;
  bool b[400];
  cin >> n >> k >> c;
  for (int i = 1; i <= 399; i++) b[i] = false;
  for (int i = 0; i < c; i++) {
    int t;
    cin >> t;
    b[t] = true;
  }
  int q = 1;
  for (int i = 1; i <= n; i++) {
    if (q == k || b[i] == true) {
      ans++;
      q = 0;
    }
    q++;
  }
  cout << ans;
  return 0;
}
