#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define fr first
#define sc second
#define endl '\n'
#define pb push_back
#define sz(x) (int)((x).size())
#define all(x) (x).begin(), (x).end()
#define pw(x) (1ll << x)
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("unroll-loops")
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef unsigned long long ull;
typedef pair <int, int> pii;
template <typename T> bool umax (T &a, const T &b) { return (a < b ? a = b, 1 : 0); }
template <typename T> bool umin (T &a, const T &b) { return (a > b ? a = b, 1 : 0); }
template <typename T>
using oset = tree<T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
const int N = 2e5 + 100;
vector <pii> g[N];
ll deep[N], siz[N], mx[N], sum[N];
set <pii> *sub[N];
/// {sum edges from root, deep}
void calc (int v, int p, int dep, ll depp) {
siz[v] = 1; mx[v] = -1;
deep[v] = dep; sum[v] = depp;
for (auto [u, w] : g[v]) {
if (u != p) {
calc(u, v, dep + 1, depp + w);
siz[v] += siz[u];
if (mx[v] == -1 || siz[u] > siz[mx[v]]) mx[v] = u;
}
}
}
ll ans = 1e18;
void dfs (int v, int p, int k) {
for (auto [u, w] : g[v]) {
if (u != p && u != mx[v]) dfs(u, v, k);
}
if (mx[v] == -1) {
sub[v] = new set <pii> ();
}
else {
dfs(mx[v], v, k);
sub[v] = sub[mx[v]];
}
sub[v]->insert({sum[v], deep[v]});
for (auto [u, w] : g[v]) {
if (u != p && u != mx[v]) {
for (auto [sumU, deepU] : *sub[u]) {
if (sumU - sum[v] > k) continue;
auto it = (*sub[v]).lower_bound({k + sum[v] - (sumU - sum[v]), -2e9});
if (it != (*sub[v]).end() && it->fr - sum[v] + sumU - sum[v] == k) ans = min(ans, deepU - deep[v] + it->sc - deep[v]);
}
for (auto i : *sub[u]) {
sub[v]->insert(i);
}
}
}
/// vertical
auto it = (*sub[v]).lower_bound({k + sum[v], -2e9});
if (it != (*sub[v]).end() && it->fr - sum[v] == k) ans = min(ans, it->sc - deep[v]);
}
int best_path (int n, int k, int h[][2], int l[]) {
for (int i = 0; i < n - 1; ++i) {
g[h[i][0]].pb({h[i][1], l[i]});
g[h[i][1]].pb({h[i][0], l[i]});
}
calc(0, -1, 0, 0);
dfs(0, -1, k);
if (ans == 1e18) ans = -1;
return ans;
}
//signed main()
//{
// ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
// int n, k; cin >> n >> k;
// int h[n - 1][2], l[n - 1];
// for (int i = 0; i < n - 1; ++i) cin >> h[i][0] >> h[i][1];
// for (int i = 0; i < n - 1; ++i) cin >> l[i];
// cout << best_path(n, k, h, l);
// return 0;
//}
