#include <bits/stdc++.h>
using namespace std;
int n, k, c;
int a[367];
int g;
int main() {
  g = -1;
  cin >> n >> k >> c;
  a[0] = 0;
  a[c + 1] = n + 1;
  for (int i = 1; i <= c; i++) cin >> a[i];
  for (int i = 1; i <= c + 1; i++) g += 1 + (a[i] - a[i - 1] - 1) / k;
  cout << g;
  return 0;
}
