#include <bits/stdc++.h>
#define ll int
#define ld long double
#define fs first
#define sc second
using namespace std;
const ll N = 2e5 + 6;
const ll Log2 = 19;
const ll inf = 1e9 + 7;
typedef pair<ll,ll> LL;
vector<LL> g[N];
ll n,k,w[N],H[N][2],child[N],len[N],d[N],ans = inf,L[N];
bool Is_cent[N];
multiset<LL> All;
ll Find_cent(ll u,ll p,ll total){
for (auto i : g[u]){
ll v = i.fs,now = i.sc;
if (!Is_cent[v]&&v != p&&child[v]*2 >= total) return Find_cent(v,u,total);
}
Is_cent[u] = 1; return u;
}
void reDFS(ll u,ll p){
child[u] = 1;
for (auto i : g[u]){
ll v = i.fs,now = i.sc;
if (!Is_cent[v] && v != p){
len[v] = len[u] + now; d[v] = d[u] + 1;
reDFS(v,u); child[u] += child[v];
}
}
}
void Add(ll u,ll p){
All.insert({len[u],d[u]});
for (auto i : g[u]){
ll v = i.fs;
if (!Is_cent[v] && v != p) Add(v,u);
}
}
void Delete(ll u,ll p){
auto it = All.find({len[u],d[u]});
All.erase(it);
for (auto i : g[u]){
ll v = i.fs;
if (!Is_cent[v] && v != p) Delete(v,u);
}
}
void update(ll u,ll p){
if (len[u] <= k){
LL it = *All.find({k - len[u],0}); //cout<<it.fs<<" "<<k <<" "<< len[u]; exit(0);
if (it.fs == k - len[u]) ans = min(ans,it.sc + d[u]);
}
for (auto i : g[u]){
ll v = i.fs;
if (!Is_cent[v] && v != p) update(v,u);
}
}
void DFS(ll u){
reDFS(u,0); All.clear();
ll centroid = Find_cent(u,0,child[u]); len[centroid] = 0;
reDFS(centroid,0); u = centroid;
//cout<<len[2]; exit(0);
for (auto i : g[u]){
ll v = i.fs;
if (!Is_cent[v]) Add(v,u);
}
for (auto i : g[u]){
ll v = i.fs;
if (!Is_cent[v]){
Delete(v,u); update(v,u); Add(v,u);
}
}
for (auto i : g[u]){
ll v = i.fs;
if (!Is_cent[v]) DFS(v);
}
}
ll best_path(ll num,ll K,ll H[][2],ll L[]){
n = num; k = K;
for (ll i = 0;i < n - 1;i++){
ll x = H[i][0],y = H[i][1]; x++; y++;
g[x].push_back({y,L[i]}); g[y].push_back({x,L[i]});
}
DFS(1);
if (ans != inf) return ans;
return -1;
}
/*
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0), cout.tie(0);
#define task "test"
if (fopen(task".inp", "r")){
freopen(task".inp", "r", stdin);
//freopen(task".out", "w", stdout);
}
cin>>n>>k;
for (ll i = 0;i < n - 1;i++) cin>>H[i][0]>>H[i][1];
for (ll i = 0;i < n - 1;i++) cin>>L[i];
cout<<best_path(n,k,H,L);
}
*/
