#include <bits/stdc++.h>
using namespace std;
int h1, h2, a, b;
int solve() {
  int d = h2 - h1 - 8 * a;
  int up = (a - b) * 12;
  if (d <= 0) return 0;
  if (up <= 0) return -1;
  return d / up + !!(d % up);
}
int main() {
  cin >> h1 >> h2 >> a >> b;
  cout << solve() << "\n";
  return 0;
}
