#include <bits/stdc++.h>
using namespace std;
int main() {
  int ha, hb, a, b, res = 0, h1;
  scanf("%d %d", &ha, &hb);
  scanf("%d %d", &a, &b);
  ha += a * 8;
  h1 = ha;
  if (ha >= hb) {
    cout << 0 << endl;
    return 0;
  }
  while (true) {
    res++;
    ha -= (b * 12);
    ha += (a * 12);
    if (ha >= hb) break;
    if (ha <= h1) {
      res = -1;
      break;
    }
  }
  cout << res << endl;
  return 0;
}
