#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, l = 0, n, p, i, sum = 0;
  cin >> n >> p;
  cin >> c;
  for (int i = 0; i < c; i++) {
    cin >> a;
    sum++;
    sum += (a - l - 1) / p;
    l = a;
  }
  sum += (n - l) / p;
  cout << sum << endl;
  return 0;
}
