/*
dsu: (ds[y] - ds[x]) + (ds[z] - ds[x]) = k
ds[z] = k + 2 ds[x] - ds[y];
# edges = (dep[y] - dep[x]) + (dep[z] - dep[x])
*/
#include <iostream>
#include <algorithm>
#include <cstring>
#include <map>
using namespace std;
const int N = 200010, INF = 0x3f3f3f3f;
int res = INF;
int h[2 * N], e[2 * N], ne[2 * N], w[2 * N], idx;
map<int, int> dep[N];
void add(int a, int b, int c) {
e[idx] = b, w[idx] = c, ne[idx] = h[a], h[a] = idx++;
}
void dfs(int u, int fa, int dist, int depth, int k) {
dep[u][dist] = depth;
for (int i = h[u]; i != -1; i = ne[i]) {
int j = e[i];
if (j == fa) {
continue;
}
dfs(j, u, dist + w[i], depth + 1, k);
for (auto t : dep[j]) {
int ds = k + 2 * dist - t.first;
if (ds >= 0 && dep[u].count(ds) > 0) {
res = min(res, dep[u][ds] + dep[j][t.first] - 2 * depth);
}
}
for (auto t : dep[j]) {
if (dep[u][t.first]) dep[u][t.first] = min(dep[u][t.first], t.second);
else dep[u][t.first] = t.second;
}
}
}
int best_path(int n, int k, int he[][2], int l[]) {
for (int i = 0; i < n - 1; i++) {
add(he[i][0] + 1, he[i][1] + 1, l[i]), add(he[i][1] + 1, he[i][0] + 1, l[i]);
}
dfs(1, -1, 0, 1, k);
return (res == INF ? -1 : res);
}
