A convinent way to compute the path length of a tree is to sum, for all k, the product of k and the umber of nodes at level k.

The path length of a tree is the sum of the levels of all the tree's nodes. The path length can have simple recursive definition as follows.

The path length of a tree with N nodes is the sum of the path lengths of the subtrees of its root plus N-1.

I am not able to follow above recursive defintion. Kindly explain with simple example.