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