#include <bits/stdc++.h>
using namespace std;
using i64 = long long int;
using ii = pair<int, int>;
using ii64 = pair<i64, i64>;
int main() {
  int a, b, f, k;
  scanf("%d %d %d %d", &a, &b, &f, &k);
  int tf = a - f;
  int gas = b - f;
  if (gas < 0) {
    printf("-1");
    return 0;
  }
  int count = 0;
  for (int i = 0; i < k; i++) {
    if (i % 2 == 0) {
      if (i == k - 1) {
        if (gas < tf) {
          gas = b;
          count++;
        }
      } else {
        if (gas < tf * 2) {
          gas = b;
          count++;
        }
      }
      if (i == k - 1) {
        gas -= tf;
      } else {
        gas -= tf * 2;
      }
      if (gas < 0) {
        printf("-1");
        return 0;
      }
    } else {
      if (i == k - 1) {
        if (gas < f) {
          gas = b;
          count++;
        }
      } else {
        if (gas < f * 2) {
          gas = b;
          count++;
        }
      }
      if (i == k - 1) {
        gas -= f;
      } else {
        gas -= f * 2;
      }
      if (gas < 0) {
        printf("-1");
        return 0;
      }
    }
  }
  printf("%d", count);
  return 0;
}
