#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  int now = b;
  int ret = 0;
  for (int i = 1; i <= k; i++) {
    if (i % 2 == 1) {
      if (now < f) {
        return puts("-1"), 0;
      }
      if (i == k) {
        if (now < a) {
          ret++;
        }
        if (b < a - f) {
          return puts("-1"), 0;
        }
      } else {
        if (now < a + a - f) {
          ret++;
          now = b - (a - f);
        } else {
          now = now - a;
        }
      }
    } else {
      if (now < a - f) {
        return puts("-1"), 0;
      }
      if (i == k) {
        if (now < a) {
          ret++;
        }
        if (b < f) {
          return puts("-1"), 0;
        }
      } else {
        if (now < a + f) {
          ret++;
          now = b - f;
        } else {
          now = now - a;
        }
      }
    }
  }
  cout << ret << endl;
  return 0;
}
