#include <bits/stdc++.h>
using namespace std;
const long long INF = (1LL << 60) - 1;
bool isPowerOfTwo(long long x) { return x && (!(x & (x - 1))); }
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long h1, h2, a, b;
  cin >> h1 >> h2 >> a >> b;
  if (h1 + 8 * a >= h2)
    cout << 0 << endl;
  else if (a > b) {
    long long cal = h2 - h1 - 8 * a;
    long long cal2 = 12 * (a - b);
    long long ans = (cal + cal2 - 1) / cal2;
    cout << ans << endl;
  } else
    cout << -1 << endl;
}
