#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e6 + 5;
int main() {
  long long a, b, f, k;
  cin >> a >> b >> f >> k;
  long long now = b, ans = 0;
  if (k == 1) {
    if (b < f || b < a - f) {
      cout << -1 << endl;
      return 0;
    }
    if (now >= a) {
      cout << 0 << endl;
    } else
      cout << 1 << endl;
  } else if (k == 2) {
    if (b < f || b < 2 * (a - f)) {
      cout << -1 << endl;
      return 0;
    }
    now = b - f;
    if (now >= 2 * (a - f)) {
      now -= 2 * (a - f);
    } else {
      ans++;
      now = b - 2 * (a - f);
    }
    if (now < f) ans++;
    cout << ans << endl;
  } else {
    int mod = k % 2;
    k /= 2;
    k--;
    now -= f;
    int q1 = 2 * f, q2 = 2 * (a - f);
    if (b < q1 || b < q2) {
      cout << -1 << endl;
      return 0;
    }
    while (k--) {
      if (now >= q2) {
        now -= q2;
      } else {
        ans++;
        now = b - q2;
      }
      if (now >= q1) {
        now -= q1;
      } else {
        ans++;
        now = b - q1;
      }
    }
    if (now >= q2) {
      now -= q2;
    } else {
      ans++;
      now = b - q2;
    }
    if (mod) {
      if (now >= q1) {
        now -= q1;
      } else {
        ans++;
        now = b - q1;
      }
      if (now < a - f) {
        ans++;
      }
    } else {
      if (now < f) ans++;
    }
    cout << ans << endl;
  }
}
