#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, a, b;
  cin >> h1 >> h2 >> a >> b;
  int aday = a * 12, bday = b * 12;
  h1 += a * 8;
  if (aday <= bday && h1 < h2) {
    cout << -1 << "\n";
    return 0;
  }
  if (h1 >= h2) {
    cout << 0 << "\n";
    return 0;
  }
  int d = aday - bday;
  int res = (h2 - h1 + d - 1) / d;
  cout << res << "\n";
  return 0;
}
