#include <bits/stdc++.h>
using namespace std;
long long int a, b, c, d;
int main() {
  cin >> a >> b >> c >> d;
  if (a + 8 * c >= b)
    cout << "0";
  else if (c > d) {
    int num = b - a - 8 * c, den = 12 * (c - d);
    cout << (num + den - 1) / den;
  } else
    cout << "-1";
  return 0;
}
