#include <bits/stdc++.h>
int main(void) {
  int a[1000000];
  int n, k;
  int count = 1;
  scanf("%d %d", &n, &k);
  for (int i = 0; i < n; i++) {
    scanf("%d", &a[i]);
  }
  int s = k;
  int i = 1;
  while (n > 0) {
    if (s >= n) {
      printf("%d", i);
      break;
    }
    i = i + 1;
    s = s + k - 1;
  }
  return 0;
}
