#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  int K;
  cin >> N;
  cin >> K;
  int array[N];
  for (int i = 0; i < N; i++) {
    cin >> array[i];
  }
  sort(array, array + N);
  int min;
  int min_num = 1;
  min = array[0];
  for (int i = 1; i < N; i++) {
    if (array[i] == min)
      min_num++;
    else
      break;
  }
  if (((N - min_num) / (K - 1)) == 0) {
    cout << (N - min_num) / (K - 1) << endl;
  } else {
    cout << (N - min_num) / (K - 1) + 1 << endl;
  }
}
