#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define mp make_pair
#define pb push_back
#define li(i,a,b) for(int (i) = (a); (i)<(b); i++)
#define all(v) v.begin(), v.end()
#define mem(name, val) memset(name, val, sizeof(name))
#define endl '\n'
const int nmax = 2e5+3;
const int kmax = 1e6+3;
int n,k,ans, cnt[kmax], sz[nmax], r[nmax];
vector<pii> adj[nmax];
int dfs_sz(int v, int p = -1){
sz[v]=1;
for(auto [u,len] : adj[v]){
if(u == p || r[u]) continue;
sz[v] += dfs_sz(u,v);
}
return sz[v];
}
int get_center(int v, int ts, int p = -1){
for(auto [u,len] : adj[v]){
if(u == p || r[u]) continue;
if(sz[u] * 2 > n) return get_center(u,ts,v);
}
return v;
}
void get_cnt(int sum, bool filling, int u, int p = -1, int depth = 1)
{
if(sum > k) return;
if(filling)
cnt[sum] = min(cnt[sum],depth);
else {
if(cnt[k - sum] != -1){
if(ans == -1) ans = depth+cnt[k-sum];
else ans = min(ans, depth+cnt[k-sum]);
}
}
for(auto [v, len]: adj[u]) {
if(v == p || r[v]) continue;
get_cnt(sum + len, filling, v, u, depth + 1);
}
}
void del(int sum, int u, int p = -1)
{
if(sum > k) return;
cnt[sum] = -1;
for(auto [v,len]: adj[u]) {
if(v==p || r[v]) continue;
del(sum + len, v, u);
}
}
void build_decomp(int v = 0){
int center = get_center(v, dfs_sz(v));
r[center] = 1;
cnt[0]=0;
for(auto [u,len] : adj[v]){
if(r[u]) continue;
get_cnt(len,false,v);
get_cnt(len,true,v);
}
for(auto [u,len]: adj[v]){
if(r[v])continue;
del(len, u);
}
for(auto [u,len] : adj[v]){
if(r[u]) continue;
build_decomp(u);
}
}
int best_path(){
ans = -1;
mem(cnt,-1);
build_decomp(0);
return ans;
}
int main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL);
cin>>n>>k;
vector<vector<int>> h(n, vector<int>(2));
vector<int> l(n);
li(i,0,n-1){
cin>>h[i][0]>>h[i][1];
}
li(i,0,n-1){
cin>>l[i];
}
li(i,0,n-1){
adj[h[i][0]].pb({h[i][1],l[i]});
adj[h[i][1]].pb({h[i][0],l[i]});
}
cout<<best_path();
return 0;
}
