http://codeforces.com/contest/789/problems

#### D. Weird journey

Announcement: Note that it is not necessary for good path to go through all cities, we care only about roads.

1. 两条不相交的边，有四个点的度数为奇数，不满足欧拉路
2. 两条有公共顶点的边，满足有两个点的度数为奇数
3. 一个环一条边，满足有两个点的度数为奇数
4. 两个自环，所有点的度数均为偶数，满足欧拉回路