#include <bits/stdc++.h>
using namespace std;
int main() {
  int days, mini, holidays, counter = 0, counter_ayam = 0;
  cin >> days >> mini >> holidays;
  int x[holidays];
  bool p = false;
  for (int i = 0; i < holidays; i++) cin >> x[i];
  if (holidays > 0) p = true;
  for (int i = 1, a = 0; i <= days; i++) {
    counter_ayam++;
    if (p && i == x[a]) {
      a++;
      counter++;
      counter_ayam = 0;
      if (a == holidays) p = false;
    }
    if (counter_ayam == mini) {
      counter++;
      counter_ayam = 0;
    }
  }
  cout << counter << endl;
}
