Why do problem setters make a lot of stories for problems which are useless ? for example in problem 1292C - Xenon's Attack on the Gangs, the statement says " It's guaranteed that links are placed in such a way that each pair of gangs will be connected by exactly one simple path. " , in the other line says "This network contains exactly n−1 direct links, each of them connecting two gangs together. The links are placed in such a way that every pair of gangs is connected through a sequence of direct links." , which are properties of tree, and can just say the graph is a tree. I think the whole statement can be like : "You are given a tree of n vertices, What is maximum value of sum of mex(u, v) for 1<=u<=v<=n (I don't know how to write sigma ), If we assign each edge an integer from 0 to n-2 and each number should be assigned to exactly one edge and mex(u, v) return smallest non-negative integer that does not appear on any edge on simple path between u and v." I think if a person doesn't know what is a tree, He will not be able to solve it. Do stories have benefits for you ? are they fun for you ? ( sorry for my poor English )