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