#include <bits/stdc++.h>
using namespace std;
typedef pair<long long,long long> ii;
#define ff first
#define ss second
const long long maxn = 1e6 + 10;
map<long long,long long> mp;
long long n;
long long k;
vector <ii> adj[maxn];
long long s[maxn],dau[maxn];
long long ans = 1e9;
long long get_sz(long long x, long long p)
{
s[x]=1;
for (ii i:adj[x])
if (i.ff!=p&&!dau[i.ff]) s[x]+=get_sz(i.ff,x);
return s[x];
}
long long find_centroid(long long x, long long p, long long sz)
{
for (ii i:adj[x])
if (i.ff!=p&&!dau[i.ff]&&s[i.ff]*2>sz) return find_centroid(i.ff,x,sz);
return x;
}
void dfs(long long x, long long p, long long val, long long depth)
{
if (val<=k&&mp[k-val]) ans=min(ans,depth+mp[k-val]);
if (val==k) ans=min(ans,depth);
for (ii i:adj[x])
if (i.ff!=p&&!dau[i.ff]) dfs(i.ff,x,1LL*val+i.ss,depth+1);
}
void dfs2(long long x, long long p, long long val, long long depth)
{
if (val<=k)
{
if (!mp[val]) mp[val] = depth;
else mp[val]=min(mp[val],depth);
}
for (ii i:adj[x])
if (i.ff!=p&&!dau[i.ff]) dfs2(i.ff,x,1LL*val+i.ss,depth+1);
}
void decompose(long long x)
{
x=find_centroid(x,x,get_sz(x,x));
dau[x]=1;
mp.clear();
for (ii i:adj[x])
if (!dau[i.ff])
{
dfs(i.ff,x,1LL*i.ss,1);
dfs2(i.ff,x,1LL*i.ss,1);
}
for (ii i:adj[x])
if (!dau[i.ff]) decompose(i.ff);
}
ii e[maxn];
int best_path(int N, int K, int H[][2], int L[])
{
n=N; k=K;
for (long long i=1; i<=n; i++)
{
adj[i].clear();
dau[i]=0;
}
ans=1e9;
for (long long i=0; i<n-1; i++)
{
e[i].ff=H[i][0];
e[i].ss=H[i][1];
}
for (long long i=0; i<n-1; i++)
{
long long w=L[i];
long long u=e[i].ff; long long v=e[i].ss;
u++; v++;
adj[u].push_back({v,w});
adj[v].push_back({u,w});
}
decompose(1);
if (ans==1e9) ans=-1;
return ans;
}
