#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c;
  cin >> n >> k >> c;
  int a[c + 1];
  for (int i = 1; i <= c; i++) {
    cin >> a[i];
  }
  a[c + 1] = 366;
  int gift = 0;
  int i = k;
  int j = 1;
  while ((i <= n)) {
    if (i >= a[j]) {
      i = a[j];
      j++;
    }
    gift++;
    i = i + k;
    if ((i > n) && (a[j] <= n)) {
      i = a[j];
    }
  }
  cout << gift;
  return 0;
}
