#include <bits/stdc++.h>
using namespace std ;
//#define int int64_t //be careful about this
#define endl "\n"
#define f(i,a,b) for(int i=int(a);i<int(b);++i)
#define pr pair
#define ar array
#define fr first
#define sc second
#define vt vector
#define pb push_back
#define eb emplace_back
#define LB lower_bound
#define UB upper_bound
#define PQ priority_queue
#define sz(x) ((int)(x).size())
#define all(a) (a).begin(),(a).end()
#define allr(a) (a).rbegin(),(a).rend()
#define mem(a,b) memset(a, b, sizeof(a))
template<class A> void rd(vt<A>& v);
template<class T> void rd(T& x){ cin >> x; }
template<class H, class... T> void rd(H& h, T&... t) { rd(h) ; rd(t...) ;}
template<class A> void rd(vt<A>& x) { for(auto& a : x) rd(a) ;}
template<class T> bool ckmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
template<typename T>
void __p(T a) {
cout<<a;
}
template<typename T, typename F>
void __p(pair<T, F> a) {
cout<<"{";
__p(a.first);
cout<<",";
__p(a.second);
cout<<"}\n";
}
template<typename T>
void __p(std::vector<T> a) {
cout<<"{";
for(auto it=a.begin(); it<a.end(); it++)
__p(*it),cout<<",}\n"[it+1==a.end()];
}
template<typename T, typename ...Arg>
void __p(T a1, Arg ...a) {
__p(a1);
__p(a...);
}
template<typename Arg1>
void __f(const char *name, Arg1 &&arg1) {
cout<<name<<" : ";
__p(arg1);
cout<<endl;
}
template<typename Arg1, typename ... Args>
void __f(const char *names, Arg1 &&arg1, Args &&... args) {
int bracket=0,i=0;
for(;; i++)
if(names[i]==','&&bracket==0)
break;
else if(names[i]=='(')
bracket++;
else if(names[i]==')')
bracket--;
const char *comma=names+i;
cout.write(names,comma-names)<<" : ";
__p(arg1);
cout<<" | ";
__f(comma+1,args...);
}
void setIO(string s = "") {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin.exceptions(cin.failbit);
cout.precision(15); cout << fixed;
if(sz(s)){
freopen((s+".in").c_str(),"r",stdin);
freopen((s+".out").c_str(),"w",stdout);
}
}
int best_path(int N, int K, int H[][2], int L[]) {
vt<ar<int,2>> g[N];
f(i,0,N-1){
int u = H[i][0], v = H[i][1], w = L[i];
g[u].pb({v,w});
g[v].pb({u,w});
}
int ans = N;
map<int,int> dp[N];
vt<int> ew(N), el(N); /* current dp[i] is actually {a + extra_weight, b + extra_length} */
function<void(int,int)> dfs = [&](int i,int p){
dp[i][0] = ew[i] = el[i] = 0;
for(auto [j,w] : g[i]) if(j != p){
dfs(j,i);
if(sz(dp[i]) > sz(dp[j])){
// update ans.
for(auto [w_j, l_j] : dp[j]){
int actual_w = w_j + ew[j] + w;
int actual_l = l_j + el[j] + 1;
int need_w = K - actual_w;
int put_i = need_w - ew[i];
if(dp[i].count(put_i)){
ckmin(ans,dp[i][put_i] + el[i] + actual_l);
}
}
// update dp[i].
for(auto [w_j, l_j] : dp[j]){
int actual_w = w_j + ew[j] + w;
int actual_l = l_j + el[j] + 1;
int put_i = actual_w - ew[i];
int put_l = actual_l - el[i];
if(dp[i].count(put_i)){
ckmin(dp[i][put_i], put_l);
}else{
dp[i][put_i] = put_l;
}
}
dp[j].clear();
}else{
// update ans.
for(auto [w_i,l_i] : dp[i]){
int actual_w = w_i + ew[i] + w;
int actual_l = l_i + el[i] + 1;
int need_w = K - actual_w;
int put_j = need_w - ew[j];
if(dp[j].count(put_j)){
ckmin(ans,dp[j][put_j] + el[j] + actual_l);
}
}
//update dp[j].
el[j] += 1;
ew[j] += w;
for(auto [w_i,l_i] : dp[i]){
int actual_w = w_i + ew[i];
int actual_l = l_i + el[i];
int put_j = actual_w - ew[j];
int put_l = actual_l - el[j];
if(dp[j].count(put_j)){
ckmin(dp[j][put_j],put_l);
}else{
dp[j][put_j] = put_l;
}
}
// swap
dp[i].swap(dp[j]);
dp[j].clear();
ew[i] = ew[j];
el[i] = el[j];
}
}
};
dfs(0,0);
if(ans == N) ans = -1;
return ans;
}
/*
#include <stdio.h>
#include <stdlib.h>
#define MAX_N 500000
static int N, K;
static int H[MAX_N][2];
static int L[MAX_N];
static int solution;
inline
void my_assert(int e) {if (!e) abort();}
void read_input()
{
int i;
my_assert(2==scanf("%d %d",&N,&K));
for(i=0; i<N-1; i++)
my_assert(2==scanf("%d %d",&H[i][0],&H[i][1]));
for(i=0; i<N-1; i++)
my_assert(1==scanf("%d",&L[i]));
my_assert(1==scanf("%d",&solution));
}
int main()
{
int ans;
read_input();
ans = best_path(N,K,H,L);
if(ans==solution)
printf("Correct.\n");
else
printf("Incorrect. Returned %d, Expected %d.\n",ans,solution);
return 0;
}
3 3
0 1
1 2
1
1
-1
11 12
0 1
0 2
2 3
3 4
4 5
0 6
6 7
6 8
8 9
8 10
3
4
5
4
6
3
2
5
6
7
2
*/
