#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  int a[100009];
  cin >> n >> k;
  for (int i = 0; i < n; ++i) cin >> a[i];
  if (k == n) {
    cout << 1;
  } else {
    cout << n / (k - 1) + 1;
  }
}
