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