7 6
1 2
2 3
3 4
4 6
6 5
5 7

hack 了：

https://www.luogu.com.cn/article/aneci6an

https://www.luogu.com.cn/article/g4t3sy5m

https://www.luogu.com.cn/article/oxknnstj

https://www.luogu.com.cn/article/0yhlf858

https://www.luogu.com.cn/article/0avd2kfe

@cff_0102 @luogu_gza