in：

1
15

out：

0

死因：没判无解。

https://www.luogu.com.cn/blog/user59424/solution-p1731

https://www.luogu.com.cn/blog/user48036/solution-p1731

https://www.luogu.com.cn/blog/zqiceberg/solution-p1731

https://victorique.blog.luogu.org/solution-p1731

https://victorique.blog.luogu.org/solution-p1731

https://www.luogu.com.cn/blog/user41331/solution-p1731

https://www.luogu.com.cn/blog/user24027/solution-p1731

https://www.luogu.com.cn/blog/yhzq/solution-p1731