#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  int h;
  cin >> h;
  if (h > 0) {
    vector<int> hs(h);
    for (int i = 0; i < h; i++) {
      cin >> hs[i];
    }
    int p = 0;
    int pnt = 0;
    for (int i = k; i <= n || pnt != h;) {
      if (hs[pnt] <= i && pnt < h) {
        i = hs[pnt] + k;
        pnt++;
      } else {
        i += k;
      }
      p++;
    }
    cout << p;
  } else {
    cout << n / k;
  }
  return 0;
}
