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