#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int maxn = 1e6 + 5;
ll d[maxn], h[maxn];
vector<pair<int, int>> g[maxn];
ll ans = 1e9;
int n, k;
void dfs0(int u, int p){
for(auto [v, w]: g[u]){
if(v == p) continue;
d[v] = d[u] + w;
h[v] = h[u] + 1;
dfs0(v, u);
}
}
void merge(map<ll, ll> &a, map<ll, ll> &b, ll rh, ll rd){
if((int)a.size() < (int)b.size()) swap(a , b);
for(auto [v1, v2]: b){
if(a.count(k - v1 + 2 * rd)) ans = min(ans, a[k - v1 + 2 * rd] + v2 - 2 * rh);
}
for(auto [v1, v2]: b){
if(a.count(v1)) a[v1] = min(a[v1], v2);
else a[v1] = v2;
}
if(a.count(k + rd)) ans = min(ans, a[k + rd] - rh);
}
map<ll, ll> dfs(int u, int p){
map<ll, ll> res;
res[d[u]] = h[u];
for(auto [v, w]: g[u]){
if(v == p) continue;
map<ll, ll> f = dfs(v, u);
merge(res, f, h[u], d[u]);
}
return res;
}
int best_path(int N, int K, int H[][2], int L[]){
n = N;
k = K;
for(int i = 0; i < n - 1; i++){
int u = H[i][0], v = H[i][1], w = L[i];
g[++u].push_back({++v, w});
g[v].push_back({u, w});
}
dfs0(1, 1);
dfs(1, 1);
if(ans > n) return -1;
return ans;
}
