#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k;
  cin >> a >> b >> f >> k;
  long long r = b, ans = 0, p = 0, sum = 0;
  bool dir = true;
  while (true) {
    if (sum / a >= k) {
      cout << ans << endl;
      return 0;
    }
    long long len = 0;
    if (dir) {
      if (p < f) {
        len = (f - p);
        p = f;
      } else if (p == f) {
        if ((a - f) * 2 > r && sum + (a - f) < k * a) {
          ++ans;
          r = b;
        } else if (a - f > r) {
          ++ans;
          r = b;
        }
        len = a - f;
        p = a;
        dir = !dir;
      }
    } else {
      if (p > f) {
        len = p - f;
        p = f;
      } else if (p == f) {
        if (f * 2 > r && (sum + f) < k * a) {
          r = b;
          ++ans;
        } else if (f > r) {
          r = b;
          ++ans;
        }
        len = f;
        p = 0;
        dir = !dir;
      }
    }
    if (r < len) {
      cout << -1 << endl;
      return 0;
    }
    r -= len;
    sum += len;
  }
  return 0;
}
