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