#include<bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define pii pair<int,int>
using namespace std;
const int maxn = 2e5 + 13;
int n, k, res;
vector<pii> adj[maxn];
vector<int> del(maxn, 0), child(maxn);
map<int, int> mp;
void dfs(int u, int p) {
child[u] = 1;
for (auto x : adj[u]) {
int v = x.fi;
if (v != p && !del[v]) {
dfs(v, u);
child[u] += child[v];
}
}
}
int find_ctroid(int u, int p, int sz) {
for (auto x : adj[u]) {
int v = x.fi;
if (v != p && !del[v] && child[v] > sz / 2) {
return find_ctroid(v, u, sz);
}
}
return u;
}
void cointect(int u, int p, int depth, int dist, bool fil) {
if (dist > k) return;
if (fil) {
if (mp.find(dist) != mp.end()) {
mp[dist] = min(mp[dist], depth);
} else {
mp[dist] = depth;
}
} else {
if (mp.find(k - dist) != mp.end()) {
res = min(res, depth + mp[k - dist]);
}
}
for (auto x : adj[u]) {
int v = x.fi;
if (v != p && !del[v]) {
cointect(v, u, depth + 1, dist + x.se, fil);
}
}
}
void decompose(int u) {
dfs(u, 0);
int ct = find_ctroid(u, 0, child[u]);
del[ct] = 1;
mp.clear();
mp[0] = 0;
for (auto x : adj[ct]) {
int v = x.fi;
if (!del[v]) {
cointect(v, ct, 1, x.se, 0);
cointect(v, ct, 1, x.se, 1);
}
}
for (auto x : adj[ct]) {
int v = x.fi;
if (!del[v]) {
decompose(v);
}
}
}
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];
++u, ++v;
adj[u].pb({v, w});
adj[v].pb({u, w});
}
res = INT_MAX;
decompose(1);
return (res == INT_MAX ? -1 : res);
}
