#include <bits/stdc++.h>
using namespace std;
int h1, h2, a, b, t(14);
int main() {
  cin >> h1 >> h2 >> a >> b;
  if (h1 + a * (22. - 14.) < h2 && a <= b)
    cout << -1;
  else if (h1 + a * (22. - 14.) >= h2)
    cout << 0;
  else {
    h1 += a * (22 - 14);
    int ans(0);
    while (true) {
      ans++;
      h1 -= (12 * b);
      if (h1 + a * 12 < h2) {
        h1 += (12 * a);
      } else {
        cout << ans;
        break;
      }
    }
  }
  return 0;
}
