CF1156D 0-1-Tree
Description
You are given a tree (an undirected connected acyclic graph) consisting of $ n $ vertices and $ n - 1 $ edges. A number is written on each edge, each number is either $ 0 $ (let's call such edges $ 0 $ -edges) or $ 1 $ (those are $ 1 $ -edges).
Let's call an ordered pair of vertices $ (x, y) $ ( $ x \ne y $ ) valid if, while traversing the simple path from $ x $ to $ y $ , we never go through a $ 0 $ -edge after going through a $ 1 $ -edge. Your task is to calculate the number of valid pairs in the tree.
Input Format
N/A
Output Format
N/A
Explanation/Hint
The picture corresponding to the first example:
