#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, K, D, C, P = 0;
  cin >> N >> K >> C;
  D = K;
  int* A = new int[C];
  for (int i = 0; i < C; i++) cin >> A[i];
  for (int i = 1; i <= N; i++) {
    D--;
    if (D == 0) {
      P++;
      D = K;
    } else {
      for (int j = 0; j < C; j++)
        if (i == A[j]) {
          P++;
          D = K;
        }
    }
  }
  cout << P;
}
