#include <bits/stdc++.h>
using namespace std;
long long int m, n, tmp, q, ans, i, j, t, h1, h2, a, b;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  if (fopen("inp.txt", "r")) freopen("inp.txt", "r", stdin);
  cin >> h1 >> h2 >> a >> b;
  h1 += 8 * a;
  if (h1 >= h2) {
    cout << "0\n";
    return 0;
  }
  if (b >= a) {
    cout << "-1\n";
    return 0;
  }
  long long int t = (h2 - h1 - 1) / (12 * (a - b));
  cout << t + 1;
  return 0;
}
