#include <bits/stdc++.h>
#ifdef DEBUG
#include <debug.hpp>
#else
#define debug(...) 42
#endif
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
using namespace std;
struct Solution {
int n, k;
int ans = 1e9;
vector<vector<pair<int, int>>> g;
vector<int> sz, used;
Solution(int _n, int _k, int (*h)[2], int* l) : n(_n), k(_k) {
g.assign(n, {});
sz.assign(n, 0);
used.assign(n, 0);
for (int i = 0; i < n - 1; ++i) {
g[h[i][0]].emplace_back(h[i][1], l[i]);
g[h[i][1]].emplace_back(h[i][0], l[i]);
}
}
int find_sz(int v, int p) {
sz[v] = 1;
for (auto [to, w] : g[v]) {
if (used[to] || to == p) continue;
sz[v] += find_sz(to, v);
}
return sz[v];
}
int centr(int v, int p, int cn) {
bool flag = 1;
while (flag) {
flag = 0;
for (auto [to, w] : g[v]) {
if (used[to] || to == p) continue;
if (sz[to] > cn / 2) {
p = v;
v = to;
flag = 1;
break;
}
}
}
return v;
}
void dfs(int v, int p, int d, int l, vector<pair<int, int>> &t) {
t.emplace_back(d, l);
for (auto [to, w] : g[v]) {
if (used[to] || to == p) continue;
dfs(to, v, d + w, l + 1, t);
}
}
void root_solve(int v, int vn) {
unordered_map<int, int> d;
d.reserve(vn);
d[0] = 1;
for (auto [to, w] : g[v]) {
if (used[to]) continue;
vector<pair<int, int>> t;
t.reserve(sz[to]);
dfs(to, v, w, 1, t);
for (auto [x, y] : t) {
if (k >= x && d[k - x] != 0) {
ans = min(ans, y + d[k - x] - 1);
}
}
for (auto [x, y] : t) {
if (d[x] == 0 || d[x] > y + 1) {
d[x] = y + 1;
}
}
}
}
void decomp(int v) {
int cn = find_sz(v, v);
int c = centr(v, v, cn);
used[c] = 1;
root_solve(c, cn);
for (auto [to, w] : g[c]) {
if (used[to]) continue;
decomp(to);
}
}
int solve() {
decomp(0);
return (ans == 1e9 ? -1 : ans);
}
};
int best_path(int n, int k, int (*h)[2], int* l) {
Solution sol(n, k, h, l);
return sol.solve();
}
