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