#include <bits/stdc++.h>
using namespace std;
typedef pair<int,int> pii;
typedef long long ll;
const int MN=2e5+2;
int N,K,stsz[MN],ans=INT_MAX;
set<pii> adj[MN],minlen;
void find_stsz(int u, int p) {
stsz[u] = 1;
for(auto [v, l] : adj[u]) {
if(v == p) continue;
find_stsz(v, u);
stsz[u] += stsz[v];
}
}
int find_centroid(int u, int p, int root) {
for(auto [v, l] : adj[u]) {
if(v == p) continue;
if(stsz[v] > stsz[root]/2) return find_centroid(v, u, root);
}
return u;
}
void dfs_upd(int u, int p, int dist, int dep) {
if(dist > K) return;
auto it = minlen.lower_bound(pii(dist,0));
if(it == minlen.end() || (*it).first != dist) minlen.insert(pii(dist,dep));
else if((*it).second > dep) {
minlen.erase(it);
minlen.insert(pii(dist,dep));
}
for(auto [v, l] : adj[u]) {
if(v == p) continue;
dfs_upd(v, u, dist+l, dep+1);
}
}
void dfs_search(int u, int p, int dist, int dep) {
if(dist > K) return;
auto it = minlen.lower_bound(pii(K-dist,0));
if(it != minlen.end() && (*it).first == K-dist) ans = min(ans, (*it).second+dep);
for(auto [v, l] : adj[u]) {
if(v == p) continue;
dfs_search(v, u, dist+l, dep+1);
}
}
int best_path(int n, int k, int H[][2], int L[]) {
N = n;
K = k;
for(int i = 0; i < N-1; ++i) {
adj[H[i][0]].insert(pii(H[i][1], L[i]));
adj[H[i][1]].insert(pii(H[i][0], L[i]));
}
queue<int> q;
q.push(0);
while(!q.empty()) {
int u = q.front();
q.pop();
minlen.clear();
minlen.insert(pii(0,0));
find_stsz(u, -1);
int cen = find_centroid(u, -1, u);
for(auto [v, l] : adj[cen]) {
dfs_search(v, cen, l, 1);
dfs_upd(v, cen, l, 1);
adj[v].erase(pii(cen, l));
q.push(v);
}
}
return (ans==INT_MAX?-1:ans);
}
