Coins on a tree

Alice is playing a game on an undirected tree, the tree is made up of $N$ nodes numbered from $1$ to $N$. Each node has some coins kept on it, Alice wants to move all the coins on all nodes to a single node on the tree, to do this Alice can do the following move many times : pick any $1$ coin on a node and move that coin to its adjacent node. Alice wants to calculate the minimum number of moves n