#include <bits/stdc++.h>
using namespace std;
const int N = 222, MOD = 7 + 1e9;
int main() {
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  int now = b, ans = 0, cnt = 0;
  while (1) {
    if (now < f) {
      puts("-1");
      return 0;
    }
    now -= f;
    if (now < (a - f)) {
      ans++;
      now = b;
    }
    now -= (a - f);
    if (now < 0) {
      puts("-1");
      return 0;
    } else
      cnt++;
    if (cnt >= k) {
      cout << ans << endl;
      return 0;
    } else {
      if (now < (a - f)) {
        ans++;
        now = b - 2 * (a - f);
      } else
        now -= (a - f);
    }
    if (now < 0) {
      puts("-1");
      return 0;
    }
    if (now < f) {
      ans++;
      now = b;
    }
    now -= f;
    if (now < 0) {
      puts("-1");
      return 0;
    } else
      cnt++;
    if (cnt >= k) {
      cout << ans << endl;
      return 0;
    } else {
      if (now < f) {
        ans++;
        now = b - f;
      }
    }
  }
  return 0;
}
