#include <bits/stdc++.h>
using namespace std;
bool t(int n, int div) {
  return (div == 1) ? true : (n % div != 0) && (t(n, div - 1));
}
int main() {
  int n, k, c;
  cin >> n >> k;
  cin >> c;
  vector<int> v(c);
  for (int i = 0; i < c; i++) cin >> v[i];
  int pos = 0, ans = 0, tmp = 0;
  for (int i = 1; i <= n; i++) {
    if (c > 0 && v[pos] == i) {
      pos++;
      ans++;
      tmp = i;
    } else if (i - tmp == k) {
      tmp = i;
      ans++;
    }
  }
  cout << ans;
  return 0;
}
