#include <bits/stdc++.h>
int a, b, c, d, h;
int main() {
  scanf("%d %d %d %d", &a, &b, &c, &d);
  h = a + 8 * c;
  if (h >= b) {
    printf("0");
    return 0;
  }
  if (c <= d) {
    printf("-1");
    return 0;
  }
  printf("%d",
         (b - h) / (12 * (c - d)) + (((b - h) % (12 * (c - d)) == 0) ? 0 : 1));
  return 0;
}
