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