#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c;
  cin >> n >> k >> c;
  int arr[366] = {};
  for (int i = 0; i < c; i++) {
    int a;
    cin >> a;
    arr[a] = 1;
  }
  int last = 0;
  int limit = k;
  int count = 0;
  for (int i = 1; i <= n; i++) {
    if (arr[i] == 1 || i == limit) {
      count++;
      last = i;
      limit = i + k;
    }
  }
  cout << count;
  return 0;
}
