#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e6 + 5;
signed main() {
  long long int sum = 0;
  long long int a, b, f, k;
  cin >> a >> b >> f >> k;
  for (long long int i = b; k--; f = a - f) {
    long long int temp = 0;
    if (k) temp = a - f;
    if (i < f || b < a - f) {
      cout << "-1\n";
      return 0;
    } else if (i >= a + temp) {
      i -= a;
    } else {
      ++sum;
      i = b - (a - f);
    }
  }
  cout << sum << endl;
  return 0;
}
