#include <bits/stdc++.h>
using namespace std;
int main() {
  map<int, int> mp;
  int n, k;
  cin >> n >> k;
  int c;
  cin >> c;
  for (int i = 1; i <= c; i++) {
    int x;
    cin >> x;
    mp[x] = 1;
  }
  int ans = 0;
  int t = 0;
  for (int i = 1; i <= n; i++) {
    t++;
    if (mp[i] == 1) {
      ans++;
      t = 0;
    } else if (t == k) {
      ans++;
      t = 0;
    }
  }
  cout << ans << endl;
  return 0;
}
