#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c, p, res = 0, found = 0;
  cin >> n >> k;
  cin >> c;
  int present[10000] = {-1};
  for (int i = 1; i <= c; i++) {
    cin >> p;
    present[p] = 1;
  }
  for (int i = 1; i <= n; i++) {
    found++;
    if (found == k || present[i] == 1) {
      res++;
      found = 0;
    }
  }
  cout << res << endl;
  return 0;
}
