// #include "race.h"
#include <bits/stdc++.h>
using namespace std;
#define ll int
#define ld double
#define endl '\n'
#define fi first
#define se second
#define pb push_back
#define REP(i,r) for(ll i=0,_r=(r);i<_r;i++)
#define FOR(i,l,r) for(ll i=(l),_r=(r);i<=_r;i++)
#define FORE(i,v) for(__typeof((v).begin()) i=(v).begin();i!=(v).end();i++)
#define FORNG(i,r,l) for(ll i=(r),_l=(l);i>=_l;i--)
#define MASK(i) (1LL<<(i))
#define BIT(x,i) (((x)>>(i))&1LL)
#define all(v) (v).begin(),(v).end()
#define size(v) ((ll)(v).size())
const ll MOD = 1e9+7, N = 2e5+10, INF = 1e9, LOG = 20;
ll n,k,ans = INF;
vector<pair<ll,ll>> adj[N];
bool del[N];
ll sz[N],f[(ll)1e6 + 10];
ll get_sz(ll u,ll par = -1){
sz[u] = 1;
for(auto [v, w] : adj[u])if(v != par && !del[v])
sz[u] += get_sz(v, u);
return sz[u];
}
ll find_centroid(ll u,ll par,ll base){
for(auto [v, w] : adj[u])if(!del[v] && v != par && sz[v] * 2 >= base){
return find_centroid(v, u, base);
}
return u;
}
void dfs(ll u,ll par,ll d,ll high,ll state){
if(state == 0){
if(d <= k)ans = min(ans, high + f[k - d]);
}
if(state == 1){
if(d <= k)f[d] = min(f[d], high);
}
if(state == 2){
f[d] = INF;
}
for(auto [v, w] : adj[u]){
if(del[v] || v == par)continue;
dfs(v, u, d + w, high + 1, state);
}
}
void centroid_dcp(ll u){
ll root = find_centroid(u, -1, get_sz(u));
del[root] = 1;
f[0] = 0;
for(auto [v, w] : adj[root])if(!del[v]){
dfs(v, root, w, 1, 0);
dfs(v, root, w, 1, 1);
}
for(auto [v, w] : adj[root])if(!del[v]){
dfs(v, root, w, 1, 2);
}
for(auto [v, w] : adj[root])if(!del[v])centroid_dcp(v);
}
void solve(){
fill(f, f + 1 + k, INF);
centroid_dcp(1);
if(ans == INF)ans = -1;
}
ll best_path(ll N, ll K, ll H[][2], ll L[]){
n = N, k = K;
REP(i,N - 1){
ll u = H[i][0], v = H[i][1], w = L[i];
u++, v++;
adj[u].pb({v, w});
adj[v].pb({u, w});
}
solve();
return ans;
}
#ifdef KHOA
int main(){
ll n = 4, k = 3;
ll H[n][2], L[n];
REP(i,n - 1)cin>>L[i];
REP(i,n - 1)cin>>H[i][0]>>H[i][1];
cout<<best_path(n, k, H, L);
}
#endif
