#include <bits/stdc++.h>
//#include "race.h"
#define loop(i, a, b) for(long long i=a;i<b;i++)
#define pool(i, a, b) for(long long i=a-1;i>=b;i--)
#define fore(i, a) for(auto&& i:a)
#define fi first
#define se second
#define ps(a) push_back(a)
#define pb(a) pop_back(a)
#define sc scanf
#define vc vector
#define pa pair<ll, ll>
#define ll long long
#define lb lower_bound
#define ub upper_bound
#define all(a) a.begin(), a.end()
#define llmax LLONG_MAX/2
#define llmin -LLONG_MAX/2
using namespace std;
#define mn 200100
#define pa pair<ll, ll>
#define ld long double
ll ans=llmax, n, k, mod[mn], h[mn];
map<ll, ll> st[mn];
vc<pa> edg[mn];
void dfs(ll u, ll p){
if(p!=-1 and edg[u].size()==1){st[u][0]=h[u];return;}
ll ch=-1;
fore(v, edg[u])if(v.fi!=p){
h[v.fi]=h[u]+1;
dfs(v.fi, u);
mod[v.fi]+=v.se;
if(ch==-1 or st[v.fi].size() > st[ch].size()) ch=v.fi;
}
swap(st[ch], st[u]);swap(mod[u], mod[ch]);
if(st[u].find(k-mod[u])!=st[u].end()) ans=min(ans, st[u][k-mod[u]]-h[u]);
fore(v, edg[u]) if(v.fi!=p and v.fi!=ch){
fore(pot, st[v.fi]){
if((k-pot.fi-mod[v.fi])>0 and st[u].find((k-pot.fi-mod[v.fi])-mod[u])!=st[u].end()) ans=min(ans, pot.se+st[u][(k-pot.fi-mod[v.fi])-mod[u]]-2 * h[u]);
if((k-pot.fi-mod[v.fi])==0){ ans=min(ans, h[v.fi]-h[u]);}
}
fore(pot, st[v.fi]) if(pot.fi+mod[v.fi]<k){
if(st[u].find(pot.fi+mod[v.fi]-mod[u])==st[u].end()) st[u][pot.fi+mod[v.fi]-mod[u]]=llmax;
st[u][pot.fi+mod[v.fi]-mod[u]]=min(st[u][pot.fi+mod[v.fi]-mod[u]], pot.se);
}
}
st[u][-mod[u]]=h[u];
}
ll best_path(int N,int K, int h[][2], int l[]){
n=N, k=K;
loop(i, 0, n-1){
edg[h[i][0]].ps(make_pair(h[i][1], l[i]));
edg[h[i][1]].ps(make_pair(h[i][0], l[i]));
}
dfs(0, -1);
return (ans>n+3)? -1:ans;
}
