#include <bits/stdc++.h>
using namespace std;
#define ll long long
vector<array<ll, 2>> adj[200001];
map<ll, ll> fixd[200001];
ll dist[200001];
ll depth[200001];
ll dk = 0;
ll res = 1e18;
void dfs( ll v, ll pre){
fixd[v][dist[v]] = v;
for(auto x:adj[v]){
ll u = x[0];
ll w = x[1];
if(u == pre)continue;
dist[u] = dist[v] + w;
depth[u] = depth[v] + 1;
dfs(u, v);
if(fixd[u].size() > fixd[v].size()){
swap(fixd[u], fixd[v]);
}
for(auto [d, node]:fixd[u]){
if(node == 0)continue;
if(fixd[v].find(dk-d+2*dist[v]) != fixd[v].end()){
res = min(res, depth[node] + depth[fixd[v][dk-d+2*dist[v]]]-2*depth[v]);
}
}
for(auto [d, node]:fixd[u]){
if(fixd[v].find(d) == fixd[v].end() || depth[node] < depth[fixd[v][d]]){
fixd[v][d] = node;
}
}
fixd[u].clear();
}
}
int best_path(int n,int k,int h[][2],int L[]){
for(int i = 0; i < n-1; i++){
h[i][0]++;
h[i][1]++;
adj[h[i][0]].push_back({h[i][1], L[i]});
adj[h[i][1]].push_back({h[i][0], L[i]});
}
dk = k;
res = 1e18;
for(int i = 0; i <= n; i++){
dist[i] = 0;
depth[i] = 0;
fixd[i].clear();
}
dfs(1, -1);
if(res == 1e18)return -1;
else return res;
}
/*
int main(){
int n,k;
cin >> n >> k;
int h[n-1][2];
int L[n-1];
for(int i = 0; i < n-1; i++){
cin >> h[i][0] >> h[i][1] ;
}
for(int i = 0;i < n-1;i++)cin >> L[i];
cout << best_path(n,k,h,L) << endl;
}
*/
