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