CF29D Ant on the Tree

Connected undirected graph without cycles is called a tree. Trees is a class of graphs which is interesting not only for people, but for ants too. An ant stands at the root of some tree. He sees that there are $ n $ vertexes in the tree, and they are connected by $ n-1 $ edges so that there is a path between any pair of vertexes. A leaf is a distinct from root vertex, which is connected with exactly one other vertex. The ant wants to visit every vertex in the tree and return to the root, passing every edge twice. In addition, he wants to visit the leaves in a specific order. You are to find some possible route of the ant.

The first line contains integer $ n $ ( $ 3

If the required route doesn't exist, output -1. Otherwise, output $ 2n-1 $ numbers, describing the route. Every time the ant comes to a vertex, output it's index.