#include<bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define pb push_back
#define pii pair<int,int>
using namespace std;
const int maxn=2e5+13;
int n,k,res;
vector<pii> adj[maxn];
vector<int> del(maxn,0),child(maxn);
set<int> s;
map<int,int> mp;
void dfs(int u,int p){
child[u]=1;
for(auto x:adj[u]){
int v=x.fi;
if(v!=p&&!del[v]){
dfs(v,u);
child[u]+=child[v];
}
}
}
int find_ctroid(int u,int p,int sz){
for(auto x:adj[u]){
int v=x.fi;
if(v!=p&&!del[v]&&child[v]>sz/2){
find_ctroid(v,u,sz);
}
}
return u;
}
void collect(int u,int p,int depth,int dist, bool fil){
if(dist>k)return;
if(fil){
if(mp.find(dist)!=mp.end()){
mp[dist]=min(mp[dist],depth);
}
else{
mp[dist]=depth;
}
s.insert(dist);
}
else {
if(mp.find(k-dist)!=mp.end()){
res=min(res,depth+mp[k-dist]);
}
}
for(auto x:adj[u]){
int v=x.fi;
if(v!=p&&!del[v]){
collect(v,u,depth+1,dist+x.se,fil);
}
}
}
void decompose(int u){
dfs(u,0);
int ct=find_ctroid(u,0,child[u]);
del[ct]=1;
s.insert(0);
for(auto x:adj[ct]){
int v=x.fi;
if(!del[v]){
collect(v,ct,1,x.se,0);
collect(v,ct,1,x.se,1);
}
}
for(int x:s)mp[x]=0;
s.clear();
for(auto x:adj[u]){
int v=x.fi;
if(!del[v]){
decompose(v);
}
}
}
int best_path(int N,int K,int H[][2],int L[]){
n=N; k=K;
for(int i=0;i<n-1;++i){
int u=H[i][0],v=H[i][1],w=L[i];
++u,++v;
adj[u].pb({v,w});
adj[v].pb({u,w});
}
int res=INT_MAX;
decompose(1);
return (res==INT_MAX?-1:res);
}
