#include <bits/stdc++.h>
using namespace std;
int a[370];
int main() {
  int n, k, c, i, kol = 0, d = 0, x;
  cin >> n >> k >> c;
  for (i = 1; i <= c; i++) {
    cin >> x;
    a[x] = 1;
  }
  d = 0;
  for (i = 1; i <= n; i++) {
    d++;
    if (a[i] == 1) {
      kol++;
      d = 0;
    }
    if (d == k) {
      kol++;
      d = 0;
    }
  }
  cout << kol;
  return 0;
}
