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