Tree Walk 2

*This is the easy version of the problem. The only difference between this version and the hard version is that here, vertices can be visited by the walk at most two times.* You're given a tree (a simple connected acyclic graph) with $N$ vertices. A *walk* in the tree is a sequence of vertices $(w_1, w_2, \ldots, w_k)$ such that there exists an edge in the tree between $w_i$ and $w_{i+1}$ for