#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e2 + 7;
long long a, b, h1, h2, H, jump, req, total;
string s;
int main() {
  cin >> h1 >> h2 >> a >> b;
  req = h2 - h1, jump = (a - b) * 12, req -= (a * 8);
  if (b >= a or (h2 - h1) <= a * 8)
    puts((h2 - h1) <= a * 8 ? "0" : "-1");
  else
    cout << req / jump + (req % jump != 0) << "\n";
  return 0;
}
