#include <bits/stdc++.h>
using namespace std;
int a[400], n, c, k, ans;
int main() {
  cin >> n >> k >> c;
  for (int i = 1; i <= c; i++) {
    cin >> a[i];
    ans += (a[i] - a[i - 1] - 1) / k + 1;
  }
  ans += (n - a[c]) / k;
  cout << ans;
}
