#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k, pos = 0;
  cin >> a >> b >> f >> k;
  long long fuel = b, dest = a;
  int ans = 0;
  while (k--) {
    if (abs(pos - f) > fuel) {
      cout << -1;
      return 0;
    }
    fuel -= abs(pos - f);
    pos = f;
    if (fuel >= 2 * abs(pos - dest) && k) {
      fuel -= 2 * abs(pos - dest);
      dest ^= a;
    } else if (fuel >= abs(pos - dest) && k == 0) {
    } else if (k) {
      fuel = b - 2 * abs(pos - dest);
      dest ^= a;
      ans++;
    } else
      fuel = b - abs(pos - dest), ans++;
    if (fuel < 0) {
      cout << -1;
      return 0;
    }
  }
  cout << ans;
}
