#include <bits/stdc++.h>
const double pi = acos(-1.0);
using namespace std;
int main() {
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  if (b < f || b < (a - f)) {
    cout << "-1";
    return 0;
  }
  if (k >= 2) {
    if (b < 2 * (a - f)) {
      cout << "-1";
      return 0;
    }
  }
  if (k >= 3) {
    if (b < 2 * f) {
      cout << "-1";
      return 0;
    }
  }
  int ans = 0;
  int cnt = b;
  for (int i = 1; i <= k; i++) {
    if (i % 2 == 1) {
      cnt -= f;
      if (i == k) {
        if (cnt < (a - f)) {
          ans++;
          cnt = b;
        }
      } else {
        if (cnt < 2 * (a - f)) {
          ans++;
          cnt = b;
        }
      }
      cnt -= (a - f);
    } else {
      cnt -= (a - f);
      if (i == k) {
        if (cnt < f) {
          ans++;
          cnt = b;
        }
      } else {
        if (cnt < 2 * f) {
          ans++;
          cnt = b;
        }
      }
      cnt -= f;
    }
  }
  cout << ans;
  return 0;
}
