#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n, k;
  int c;
  cin >> n >> k;
  cin >> c;
  vector<int> a;
  for (int i = 0; i < c; i++) {
    int x;
    cin >> x;
    a.push_back(x);
  }
  a.push_back(1000);
  sort(a.begin(), a.end());
  int ans = 0;
  int nx = k;
  int j = 0;
  for (int i = 1; i <= n; i++) {
    while (a[j] == i) {
      nx = i;
      j++;
    }
    if (nx == i) {
      ans++;
      nx += k;
    }
  }
  cout << ans;
  return 0;
}
