#include <bits/stdc++.h>
using namespace std;
int main() {
  long long m, a, b, n, d, x;
  cin >> a >> b >> n >> m;
  x = (b - a) - (8 * n);
  if (x <= 0)
    cout << "0" << endl;
  else {
    if (n <= m)
      cout << "-1" << endl;
    else {
      double y = (double)x / ((12 * n) - (12 * m));
      cout << ceil(y) << endl;
    }
  }
  return 0;
}
