#include <bits/stdc++.h>
using namespace std;
int flag[1000];
int n, cnt, ans, k;
int main() {
  cin >> n >> k >> cnt;
  for (int i = 0; i < cnt; i++) {
    int a;
    cin >> a;
    flag[a] = 1;
  }
  int a = 0;
  for (int i = 1; i <= n; i++) {
    if (flag[i]) {
      ans++;
      a = i;
    }
    if (i >= a + k) {
      ans++;
      a = a + k;
    }
  }
  cout << ans;
  return 0;
}
