A group of 2m - 1 routers are interconnected in a centralized binary tree, with router at each tree node. Router I communicate with router J by sending a message to the root of the tree. The root then sends the message back down to J. Then find the mean router-router path length.

• 2 (m - 2)
• 2 (2m - 1)
• m - 1
• (2m - l)/mJ