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