The Depth Echo

In a rooted tree (root is node 0), an 'Echo' at distance D from node U is any descendant of U that is exactly D edges away. Given a tree and an integer D, calculate the total number of such (U, V) pairs in the tree where V is an Echo of U at distance D. Input Format: - Line 1: N and D. - Next N-1 lines: Two integers u and v (where u is the parent of v). Output Format: - A single integer representing the total count of pairs. Example: Input: 4 1 0 1 0 2 1 3 Output: 3 Explanation: Pairs at distance 1: (0,1), (0,2), (1,3). Total 3.