My AC code:

input:
>   3 3
>   1 2 1
>   1 2 1
>   1 3 1
>   -1

output:
>   No solution.

Why not
>   1 2

There are two roads connecting "1" and "2", so the route could be 1 - 2 - 1, with different road 1 - 2 and 2 -1

"Each sightseeing route is a sequence of road numbers y1, …, yk, k > 2."

Oops.
Sorry for bothering!