#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, a, b;
  cin >> h1 >> h2 >> a >> b;
  if (h2 - h1 <= 8 * a) {
    cout << 0 << endl;
  } else if (b >= a) {
    cout << -1 << endl;
  } else {
    bool divisible = (h2 - h1 - 8 * a) % (12 * a - 12 * b) == 0;
    cout << (h2 - h1 - 8 * a) / ((a - b) * 12) + ((divisible) ? 0 : 1) << endl;
  }
  return 0;
}
