Given a **complete** binary tree, count the number of nodes.

**Note: **

**Definition of a complete binary tree from Wikipedia:**

In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2^{h} nodes inclusive at the last level h.

**Example:**

Input:1 / \ 2 3 / \ / 4 5 6Output:6