#include <bits/stdc++.h>
using namespace std;
int h1, h2, a, b, ans;
const int INF = 10e9;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> h1 >> h2 >> a >> b;
  h1 += (a * 8);
  if (h1 >= h2) {
    cout << 0;
    return 0;
  }
  h1 -= (b * 12);
  if (h1 + (a * 12) < h2 && a <= b) {
    cout << -1;
    return 0;
  }
  ans++;
  while (h1 + (a * 12) < h2) {
    ans++;
    h1 += (a * 12);
    if (h1 >= h2) {
      cout << ans;
      return 0;
    }
    h1 -= (b * 12);
  }
  cout << ans;
}
