#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k, g, r = 0, ans = 0;
  cin >> a >> b >> f >> k;
  g = b;
  for (int i = 1; i <= k; i++) {
    if (g < abs(f - r)) {
      cout << "-1";
      return 0;
    }
    g = g - abs(f - r);
    if (i != k) {
      if (g < (2 * (abs(a - f)))) {
        g = b;
        ans++;
      }
      if (g < (2 * (abs(a - f)))) {
        cout << "-1";
        return 0;
      }
      g = g - abs(a - f);
      swap(r, a);
    } else {
      if (g < abs(a - f)) {
        g = b;
        ans++;
      }
      if (g < abs(a - f)) {
        cout << "-1";
        return 0;
      }
    }
  }
  cout << ans;
}
