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