test 4 failed
input:
8
1 2
1 3
1 4
3 5
5 6
6 7
1 8
expected:
1 2
2 3
3 4
4 5
5 6
6 7
7 8
got:
1 2
2 3
3 4
4 5
5 6
6 7
6 8