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