#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k;
  scanf("%lld %lld %lld %lld", &a, &b, &f, &k);
  long long ans = 0;
  long long cfuel = b;
  for (int i = 0; i < k - 1; i++) {
    if (i % 2 == 0) {
      if (cfuel >= 2 * a - f) {
        cfuel -= a;
      } else if (cfuel >= f && b >= a - f) {
        ans++;
        cfuel = b - (a - f);
      } else {
        printf("-1\n");
        return 0;
      }
    } else {
      if (cfuel >= a + f) {
        cfuel -= a;
      } else if (cfuel >= a - f && b >= f) {
        ans++;
        cfuel = b - f;
      } else {
        printf("-1\n");
        return 0;
      }
    }
  }
  if (k % 2) {
    if (cfuel >= a) {
    } else if (cfuel >= f && b >= a - f) {
      ans++;
    } else {
      printf("-1\n");
      return 0;
    }
  } else {
    if (cfuel >= a) {
    } else if (cfuel >= a - f && b >= f) {
      ans++;
    } else {
      printf("-1\n");
      return 0;
    }
  }
  printf("%lld\n", ans);
}
