#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, K, C;
  cin >> N >> K >> C;
  vector<int> hol(C + 2);
  hol[0] = 0;
  hol[C + 1] = N + 1;
  for (int i = 1; i <= C; i++) {
    cin >> hol[i];
  }
  int ans(0);
  for (int i = 0; i < C + 1; i++) {
    ans += (hol[i + 1] - hol[i] - 1) / K;
  }
  ans += C;
  cout << ans << endl;
  return 0;
}
