#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie();
  bool flag[1000];
  memset(flag, false, sizeof(flag));
  int n, k;
  cin >> n >> k;
  int c, x;
  cin >> c;
  for (int i = 0; i < c; i++) {
    cin >> x;
    flag[x] = true;
  }
  int now = 0;
  while (now <= n) {
    bool ketemu = false;
    for (int i = now + 1; i <= now + k; i++) {
      if (flag[i]) {
        now = i;
        ketemu = true;
        break;
      }
    }
    if (ketemu)
      continue;
    else
      flag[now + k] = true, now += k;
  }
  int cnt = 0;
  for (int i = 1; i <= n; i++) {
    if (flag[i]) cnt++;
  }
  cout << cnt << endl;
  return 0;
}
