#include <bits/stdc++.h>
using namespace std;
int h1, h2, a, b;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  cin >> h1 >> h2 >> a >> b;
  if (h1 + 8 * a >= h2) return cout << 0, 0;
  if (a <= b) return cout << -1, 0;
  cout << (h2 - h1 - 8 * a + 12 * a - 12 * b - 1) / (12 * a - 12 * b);
}
