#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0), cin.tie(0);
  int a, b, f, k, t = 0, lo = 0, ans = 0;
  cin >> a >> b >> f >> k;
  int pos[2], gas = b;
  pos[0] = 0;
  pos[1] = a;
  while (t != k) {
    if (abs(f - pos[lo]) > gas) {
      cout << -1;
      return 0;
    }
    gas -= abs(f - pos[lo]);
    if (t != k - 1 && 2 * abs(f - pos[(lo + 1) % 2]) > gas) {
      ans++;
      gas = b;
    } else if (t == k - 1) {
      if (abs(f - pos[(lo + 1) % 2]) > gas) {
        ans++;
        gas = b;
      }
    }
    gas -= abs(f - pos[(lo + 1) % 2]);
    lo = (lo + 1) % 2, t++;
  }
  if (gas >= 0)
    cout << ans;
  else
    cout << -1;
  return 0;
}
