#include <bits/stdc++.h>
const int maxn = 1e5 + 7;
int ans = 0;
int main() {
  int h1, h2, a, b;
  scanf("%d%d%d%d", &h1, &h2, &a, &b);
  int dis = h2 - h1;
  if (a * 8 >= dis)
    puts("0");
  else {
    ans++;
    h1 += a * 8 - b * 12;
    h1 += a * 12;
    if (h1 > h2)
      puts("1");
    else if (b >= a)
      puts("-1");
    else {
      h1 -= b * 12;
      dis = h2 - h1;
      int v = (a - b) * 12;
      if (a * 12 >= dis)
        puts("2");
      else {
        dis -= a * 12;
        int T = dis / v + (dis % v != 0);
        ans = ans + 1 + T;
        printf("%d\n", ans);
      }
    }
  }
}
