#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long h1, h2;
  cin >> h1 >> h2;
  long long a, b;
  cin >> a >> b;
  h1 += 8 * a;
  if (h1 >= h2) {
    cout << 0 << endl;
    return 0;
  }
  h1 -= 12 * b;
  for (long long i = 1; i <= 1e6; i++) {
    h1 += 12 * a;
    if (h1 >= h2) {
      cout << i << endl;
      return 0;
    }
    h1 -= 12 * b;
  }
  cout << -1 << endl;
  return 0;
}
