#include <bits/stdc++.h>
int main(int argc, char *argv[]) {
  long int h1, h2, a, b, number = 1;
  scanf("%ld %ld", &h1, &h2);
  scanf("%ld %ld", &a, &b);
  int night = 12, day = 8;
  if (day * a >= h2 - h1)
    printf("0\n");
  else if (a <= b)
    printf("-1\n");
  else {
    while (a * day + number * (a - b) * night < h2 - h1) number++;
    printf("%ld \n", number);
  }
  return 0;
}
