#include <bits/stdc++.h>
using namespace std;
int main() {
  int h1, h2, a, b;
  while (cin >> h1 >> h2 >> a >> b) {
    int dis = h2 - h1;
    int day = 0;
    if (b > a && dis > (a * 8)) {
      cout << -1 << endl;
      continue;
    } else if (a == b && (dis > (a * 8))) {
      cout << -1 << endl;
      continue;
    }
    while (1) {
      h1 += (a * 8);
      if (h1 >= h2) break;
      day++;
      h1 -= (b * 12);
      h1 += (a * 4);
    }
    cout << day << endl;
  }
}
