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