#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k;
  int ans = 0;
  cin >> a >> b >> f >> k;
  if (b < f) {
    cout << -1 << endl;
    return 0;
  }
  int flag = 0;
  int l = a - f;
  int temp = b;
  while (k > 0) {
    if (flag == 0) {
      if (temp < f) {
        cout << "-1" << endl;
        return 0;
      }
      temp -= f;
      if (l > temp) {
        ans++;
        temp = b;
        if (l > temp) {
          cout << "-1" << endl;
          return 0;
        }
        k--;
        temp -= l;
      } else {
        k--;
        if (k) {
          if (temp < 2 * l) {
            temp = b;
            ans++;
            if (temp < 2 * l) {
              cout << "-1" << endl;
              return 0;
            }
          }
          temp -= l;
        }
      }
      flag = 1;
      continue;
    } else {
      if (temp < l) {
        cout << "-1" << endl;
        return 0;
      }
      temp -= l;
      if (temp < f) {
        temp = b;
        ans++;
        k--;
        temp -= f;
      } else {
        k--;
        if (k) {
          if (temp < 2 * f) {
            ans++;
            temp = b;
            if (temp < 2 * f) {
              cout << "-1" << endl;
              return 0;
            }
          }
          temp -= f;
        }
      }
      flag = 0;
      continue;
    }
  }
  cout << ans << endl;
}
