How to solve http://codeforces.me/contest/242/problem/E problem with square root decompositatio?

You are given a tree,number X and each node have a number on it.You have to find some nodes a₁,a₂,a₃ ... ,a_n such a₂ is child of a₁ ,a₃ is child of a₂,...a_n is child of a_n - 1 and a₁ + a₂ + ... + a_n closest number to X.We say x is child of y if it's in subtree with root in y. This problem was on Serbian state competition 2014. for seniors,could you give me any hints or pseudocode?