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