#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  long long int a, b, c, d;
  cin >> a >> b >> c >> d;
  a += (8 * c);
  if (d >= c and b > a)
    cout << -1;
  else {
    long long int answer = 0;
    while (b > a) {
      answer++;
      a += (c - d) * 12;
    }
    cout << answer;
  }
  return 0;
}
