#include <bits/stdc++.h>
typedef long long LL;
const double INF = 1e100;
const int oo = ~0u >> 2;
const double pi = acos(-1.0);
const double EPS = 1e-8;
const int MAXN = 100033;
int h1, h2;
int a, b;
int cur;
int main() {
  scanf("%d%d", &h1, &h2);
  scanf("%d%d", &a, &b);
  if (h1 + 8 * a >= h2)
    puts("0");
  else if (a > b) {
    int num = h2 - h1 - 8 * a, l = 12 * (a - b);
    printf("%d\n", (num + l - 1) / l);
  } else
    puts("-1");
  return 0;
}
