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