Root the Tree

A *directed tree* is a directed graph such that if all edges were undirected, this graph would be a tree. A *rooted directed tree* is a directed tree in which there is one vertex (the *root*, let's denote it by $r$) such that it is possible to reach all vertices of the graph from $r$ by moving along the directed edges. You are given a directed tree with $N$ vertices (numbered $1$ through $N$).