#include <bits/stdc++.h>
int hol[400];
using namespace std;
int main() {
  int n, k, c;
  while (cin >> n >> k) {
    memset(hol, 0, sizeof(hol));
    cin >> c;
    int sum = 0;
    sum += c;
    if (c != 0) {
      cin >> hol[0];
      if (hol[0] - 1 >= k) {
        sum += (hol[0] - 1) / k;
      }
      for (int i = 1; i < c; i++) {
        cin >> hol[i];
        if (hol[i] - hol[i - 1] - 1 >= k) {
          sum += (hol[i] - hol[i - 1] - 1) / k;
        }
      }
      if (n - hol[c - 1] >= k) {
        sum += (n - hol[c - 1]) / k;
      }
    } else {
      sum += n / k;
    }
    cout << sum << endl;
  }
  return 0;
}
