#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, K;
  cin >> N >> K;
  int C;
  cin >> C;
  int res = C;
  int prev = 0;
  for (int i = 0; i < C; ++i) {
    int cur;
    cin >> cur;
    res += (cur - prev - 1) / K;
    prev = cur;
  }
  res += (N - prev) / K;
  cout << res << endl;
  return 0;
}
