hack:

input:

3 2
1 2
2 1
1 3

output:

-1

hack 的题解：

https://www.luogu.com.cn/blog/GamerDiaosi/solution-p2296 https://www.luogu.com.cn/blog/LitShone/solution-p2296 https://www.luogu.com.cn/blog/suuuuy/solution-p2296 https://www.luogu.com.cn/blog/shajjl/solution-p2296 https://www.luogu.com.cn/blog/lcezych/solution-p2296 https://www.luogu.com.cn/blog/user36788/solution-p2296 https://yizimiyuanxin.blog.luogu.org/solution-p2296 https://www.luogu.com.cn/blog/zhr666/solution-p2296 https://www.luogu.com.cn/blog/milkcandy/solution-p2296 https://www.luogu.com.cn/blog/cowcowcow/solution-p2296 https://www.luogu.com.cn/blog/Kesdiael3/solution-p2296 https://www.luogu.com.cn/blog/caolaoshi/solution-p2296 https://www.luogu.com.cn/blog/user56787/solution-p2296 https://www.luogu.com.cn/blog/liuwenyao2003/p2296-xun-zhao-dao-lu-ti-xie https://www.luogu.com.cn/blog/ctyer/noip2014t-xun-zhao-dao-lu

另外问一下这种 NOIp 的历年真题可以添加 hack 数据到题目测试里吗（我们今天下午比赛有 6 个人 AC 了这题 4 个人都过不了这个 hack 数据）