#include <bits/stdc++.h>
int main() {
  int a, b, f, k;
  scanf("%d %d %d %d", &a, &b, &f, &k);
  int left = b, res = 0;
  for (int i = 1; i <= k; i++, f = a - f) {
    int sp = a;
    if (i < k) {
      sp += (a - f);
    }
    if (sp <= left) {
      left -= a;
    } else {
      if (f <= left) {
        res++;
        left = b - (a - f);
      } else {
        printf("-1");
        return 0;
      }
    }
    if (left < 0) {
      printf("-1");
      return 0;
    }
  }
  printf("%d", res);
  return 0;
}
