I was doing well with SPOJ Problems, but due to some Problems, I am stuck with them. I am in the first year of my BTECH, but I am pretty much confident with the Ad-Hoc Problems. I am new with Graph Theory. I want to learn it.

Problem Link is this

Some one please make me understand the Problem. I have read about Trees, but not so much helped. Someone please be kind and answer. Thanks in Advance! :)