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