#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, sum = 0, last = 0;
  cin >> n >> k;
  int c;
  cin >> c;
  for (int i = 0; i < c; ++i) {
    int x;
    cin >> x;
    while (x - last > k) {
      ++sum;
      last = last + k;
      if (last > n) break;
    }
    if (last > n) break;
    ++sum, last = x;
  }
  int x = n;
  while (x - last >= k) {
    ++sum;
    last = last + k;
    if (last > n) break;
  }
  cout << sum;
  return 0;
}
