#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, tmp, res = 0;
  cin >> n >> m;
  vector<int> vec;
  vec.push_back(0);
  cin >> tmp;
  while (cin >> tmp) {
    vec.push_back(tmp);
  }
  for (int i = 0; i < ((long long)vec.size()) - 1; i++)
    if (vec[i] != vec[i + 1]) {
      res += 1 + (vec[i + 1] - vec[i] - 1) / m;
    }
  res += (n - vec[((long long)vec.size()) - 1]) / m;
  cout << res;
}
