#include <bits/stdc++.h>
using namespace std;
int day[400];
int n, k, c, ans;
int main() {
  cin >> n >> k >> c;
  for (int i = 1; i <= c; i++) {
    cin >> day[i];
    ans += (day[i] - day[i - 1] - 1) / k + 1;
  }
  ans += (n - day[c]) / k;
  cout << ans;
  return 0;
}
