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