#include <bits/stdc++.h>
using namespace std;
int main() {
  int now, aim;
  int sun, moon;
  int day = 0;
  cin >> now >> aim;
  cin >> sun >> moon;
  if (now + 8 * sun >= aim) {
    cout << day << endl;
  } else {
    if (moon >= sun) {
      cout << "-1" << endl;
    } else {
      aim = aim - now - 8 * sun;
      if (aim % (12 * (sun - moon)) == 0) {
        day = aim / (12 * (sun - moon));
      } else {
        day = aim / (12 * (sun - moon)) + 1;
      }
      cout << day << endl;
    }
  }
  return 0;
}
