#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  int h1, h2;
  cin >> h1 >> h2;
  cin >> a >> b;
  if (((h2 - h1) / (float)a) <= 8) {
    cout << 0 << endl;
  } else {
    if (a <= b) {
      cout << -1 << endl;
    } else {
      cout << ceil((h2 - h1 - a * 8.0) / (a - b) / 12) << endl;
    }
  }
  return 0;
}
