#include <bits/stdc++.h>
using namespace std;
bool passed[1001];
int main() {
  int n, k;
  cin >> n >> k;
  int c;
  cin >> c;
  for (int j = 0; j < c; j++) {
    int t;
    cin >> t;
    passed[t] = true;
  }
  int prv = 0;
  int cnt = 0;
  for (int i = 1; i <= n; i++) {
    if (passed[i] || i - prv == k) {
      prv = i;
      cnt++;
    }
  }
  cout << cnt << endl;
  return 0;
}
