#include <bits/stdc++.h>
using namespace std;
int a[100000 + 1];
int main() {
  long long h1, h2, a, b;
  scanf("%lld%lld", &h1, &h2);
  scanf("%lld%lld", &a, &b);
  h1 += 8 * a;
  if (h1 >= h2) {
    printf("0\n");
  } else if (a > b) {
    printf("%lld\n", (h2 - h1 + 12 * (a - b) - 1) / (12 * (a - b)));
  } else {
    printf("-1\n");
  }
  return 0;
}
