#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
const int MOD = 1e9 + 7;
const int N = 0;
int h1, h2, up, down;
int solve() {
  int d = h2 - h1 - 8 * up;
  if (d <= 0) return 0;
  if (down >= up) return -1;
  int inc = (up - down) * 12;
  return (d + inc - 1) / inc;
}
int main() {
  cin >> h1 >> h2 >> up >> down;
  cout << solve();
}
