#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define fi first
#define se second
#define pii pair<ll,ll>
const int N = 200005;
const int inf = 1e9;
ll del[N], d[N], ans, child[N];
vector<pii> adj[N];
map<ll, ll> mp;
int n, k;
void cnt_child(ll u, ll parent) {
child[u] = 1;
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
cnt_child(v.fi, u);
child[u] += child[v.fi];
}
}
ll centroid(ll u, ll parent, ll n) {
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
if (child[v.fi] > n / 2) {
return centroid(v.fi, u, n);
}
}
return u;
}
void dfs(ll u, ll parent, ll dep) {
if (mp.find(k - d[u]) != mp.end()) {
ans = min(ans, dep + mp[k - d[u]]);
}
if (d[u] == k) ans = min(ans, dep);
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
d[v.fi] = d[u] + v.se;
if (d[v.fi] > k) continue;
dfs(v.fi, u, dep + 1);
}
}
void dfs2(ll u, ll parent, ll dep) {
if (mp.find(d[u]) == mp.end()) {
mp[d[u]] = dep;
}
else {
mp[d[u]] = min(mp[d[u]], dep);
}
for (auto v : adj[u]) {
if (v.fi == parent || del[v.fi]) continue;
d[v.fi] = d[u] + v.se;
if (d[v.fi] > k) continue;
dfs2(v.fi, u, dep + 1);
}
}
void gq(ll x) {
mp.clear();
d[x] = 0;
for (auto v : adj[x]) {
if (del[v.fi]) continue;
d[v.fi] = d[x] + v.se;
dfs(v.fi, x, 1);
dfs2(v.fi, x, 1);
}
}
void solve(ll u) {
cnt_child(u, u);
ll x = centroid(u, u, child[u]);
gq(x);
del[x] = 1;
for (auto v : adj[x]) {
if (del[v.fi]) continue;
solve(v.fi);
}
}
int best_path(int N_input, int K_input, int H[][2], int L[]) {
n = N_input;
k = K_input;
// reset global data
for (int i = 0; i <= n; i++) {
adj[i].clear();
del[i] = d[i] = child[i] = 0;
}
ans = inf;
// build tree
for (int i = 0; i < n - 1; i++) {
int u = H[i][0] + 1;
int v = H[i][1] + 1;
int w = L[i];
adj[u].push_back({v, w});
adj[v].push_back({u, w});
}
solve(1);
if (ans == inf) return -1;
return ans;
}
