#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, res;
  int a[400], c;
  while (cin >> n >> k) {
    cin >> c;
    for (int i = 0; i < c; i++) cin >> a[i];
    sort(a, a + c);
    res = 0;
    for (int t = 0, ci = 0; t < n; res++) {
      if (ci < c && a[ci] <= t + k && a[ci] <= n) {
        t = a[ci++];
      } else {
        t += k;
        if (t > n) res--;
      }
    }
    cout << res << endl;
  }
  return 0;
}
