#include <bits/stdc++.h>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l,int r){return l+rng()%(r-l+1);}
#define fasty ios_base::sync_with_stdio(0),cin.tie(0);
#define forinc(a,b,c) for(int a=b,_c=c;a<=_c;++a)
#define fordec(a,b,c) for(int a=b,_c=c;a>=_c;--a)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ii pair<int,int>
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f, x) memset(f, x, sizeof(f))
#define bit(x,i) ((x>>(i-1))&1)
#define on(x,i) (x|(1ll<<(i-1)))
#define off(x,i) (x&~(1<<(i-1)))
#define gg exit(0);
//#define unx 1
#ifndef unx
#include "race.h"
#endif
const int N=200010;
int n,cnt,k,ret;
int sz[N],dd[N],f[N*5];
vector<ii> val;
vector<ii> ad[N];
void dfs1(int u,int p=-1){
sz[u]=1; cnt++;
forv(v,ad[u]) if(!dd[v.fi] && v.fi!=p)
dfs1(v.fi,u), sz[u]+=sz[v.fi];
}
int ftr(int u,int p=-1){
forv(v,ad[u]) if(!dd[v.fi] && v.fi!=p && sz[v.fi]>cnt/2) return ftr(v.fi,u);
return u;
}
void dfs2(int u,int p,int c,int d){
if(c>k) return;
val.push_back({c,d});
forv(v,ad[u]) if(!dd[v.fi] && v.fi!=p && v.se+c<=k)
dfs2(v.fi,u,c+v.se,d+1);
}
void ctr(int u){
cnt=0; dfs1(u); dd[u=ftr(u)]=1;
f[0]=0;
vector<int> reval;
forv(v,ad[u]) if(!dd[v.fi]){
dfs2(v.fi,u,v.se,1);
forv(i,val) if(f[k-i.fi]>-1) ret=min(ret,i.se+f[k-i.fi]);
forv(i,val) f[i.fi]=f[i.fi]<0 ? i.se : min(f[i.fi],i.se), reval.push_back(i.fi);
val.clear();
}
forv(i,reval) f[i]=-1;
forv(v,ad[u]) if(!dd[v.fi]) ctr(v.fi);
}
int best_path(int n,int _k,int h[][2],int l[]){ k=_k; ret=n;
forinc(i,0,n-2){
int u=h[i][0], v=h[i][1], w=l[i];
ad[u].push_back({v,w});
ad[v].push_back({u,w});
}
reset(f,-1);
ctr(0);
return ret<n ? ret : -1;
}
#ifdef unx
main(){
#define task "RACE"
freopen(task".inp","r",stdin);
//freopen(task".out","w",stdout);
int n, k; cin>>n>>k;
int H[100][2];
int L[100];
forinc(i,2,n){
cin>>H[i-2][0]>>H[i-2][1];
}
forinc(i,2,n){
cin>>L[i-2];
}
cout<<best_path(n,k,H,L);
}
#endif
