#include <bits/stdc++.h>
using namespace std;
vector<vector<pair<int,int>>> adj;
int n, k;
vector<int> subtr;
vector<bool> vis;
unordered_map<int,int> mp;
int ans=INT_MAX;
void findsize(int u, int p){
subtr[u]=1;
for(auto [v,w]:adj[u]){
if(v!=p and !vis[v]){
findsize(v,u);
subtr[u]+=subtr[v];
}
}
}
int findcentroid(int u, int p, int sz){
for(auto [v,w]:adj[u]){
if(v!=p and !vis[v] and subtr[v]>sz/2)return findcentroid(v,u,sz);
}
return u;
}
void dfs1(int u, int p, int dist, int depth){
if(dist>k)return;
if(mp[k-dist] or k==dist) ans=min(ans,mp[k-dist]+depth);
for(auto [v,w]:adj[u]){
if(v!=p and !vis[v]){
dfs1(v,u,dist+w,depth+1);
}
}
}
void dfs2(int u, int p, int dist, int depth){
if(dist>k)return;
if(mp[dist]==0)mp[dist]=depth;
else mp[dist]=min(mp[dist],depth);
for(auto [v,w]:adj[u]){
if(v!=p and !vis[v]){
dfs2(v,u,dist+w,depth+1);
}
}
}
void buildcentroid(int u, int p){
findsize(u,u);
int c=findcentroid(u,u,subtr[u]);
vis[c]=true;
for(auto [v,w]:adj[c]){
if(v!=p and !vis[v]){
dfs1(v,c,w,1);
dfs2(v,c,w,1);
}
}
mp.clear();
for(auto [v,w]:adj[c]){
if(!vis[v])buildcentroid(v,c);
}
}
int best_path(int N, int K, int H[][2], int L[]){
n=N;
k=K;
adj.resize(n);
subtr.resize(n);
vis.resize(n);
for(int i=0;i<n-1;i++){
adj[H[i][1]].push_back({H[i][0],L[i]});
adj[H[i][0]].push_back({H[i][1],L[i]});
}
buildcentroid(0,-1);
if(ans==INT_MAX)return -1;
return ans;
}
