#include <bits/stdc++.h>
using namespace std;
int best_path(int n, int k, int H[][2], int L[]) {
vector<vector<int>> adj(n);
for (int i = 0; i < n - 1; i++) {
int x = H[i][0];
int y = H[i][1];
adj[x].push_back(y);
adj[y].push_back(x);
}
vector<bool> visited(n);
vector<int> sz(n);
function<void(int,int)> dfs_sz = [&](int u, int p) {
sz[u] = 1;
for (auto v : adj[u]) {
if (v == p || visited[v]) continue;
dfs_sz(v, u);
sz[u] += sz[v];
}
};
vector<int> dist;
function<void(int,int,int)> dfs = [&](int u, int p, int cur) {
dist.push_back(cur);
for (auto v : adj[u]) {
if (v == p || visited[v]) continue;
dfs(v, u, cur + 1);
}
};
function<int(int,int,int)> find_centroid = [&](int u, int p, int size) {
for (auto v : adj[u]) {
if (v == p || visited[v]) continue;
if (sz[v] > size / 2)
return find_centroid(v, u, size);
}
return u;
};
vector<int> cnt(n + 1);
int ans = 0;
function<void(int)> decompose = [&](int u) {
dfs_sz(u, -1);
int centroid = find_centroid(u, -1, sz[u]);
visited[centroid] = true;
cnt[0] = 1;
for (auto v : adj[centroid]) {
if (visited[v]) continue;
dist.clear();
dfs(v, centroid, 1);
for (auto d : dist) {
if (d <= k)
ans += cnt[k - d];
}
for (auto d : dist) {
cnt[d]++;
}
}
for (int i = 0; i <= sz[u]; i++)
cnt[i] = 0;
for (auto v : adj[centroid]) {
if (!visited[v])
decompose(v);
}
};
decompose(0);
return ans;
}
