#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> ii;
const int maxn = 2e5 + 10;
vector<int> adj[maxn], cost[maxn];
ll dist[maxn]; bool vis[maxn];
int sub[maxn], len[maxn], n, k;
int ans = 1e9;
void calc(int u, int par) {
sub[u] = 1;
for(int v : adj[u]) if(v - par && !vis[v])
calc(v, u), sub[u] += sub[v];
}
int centroid(int u, int par, int r) {
for(int v : adj[u]) if(v - par && !vis[v])
if(sub[v] > r) return centroid(v, u, r);
return u;
}
vector<int> vert; int in[maxn], out[maxn];
void dfs(int u, int par) {
in[u] = vert.size(); vert.push_back(u);
for(int i = 0; i < adj[u].size(); ++i) {
int v = adj[u][i], c = cost[u][i];
if(v == par || vis[v]) continue;
dist[v] = dist[u] + c;
len[v] = len[u] + 1;
dfs(v, u);
} out[u] = vert.size() - 1;
}
void solve(int u) {
dist[u] = len[u] = 0;
vert.clear(); dfs(u, -1);
unordered_map<ll, int> mp; mp[0] = 0;
for(int v : adj[u]) if(!vis[v]) {
for(int t = in[v]; t <= out[v]; ++t) {
int w = vert[t];
if(mp.count(k - dist[w]))
ans = min(ans, mp[k - dist[w]] + len[w]);
}
for(int t = in[v]; t <= out[v]; ++t) {
int w = vert[t];
if(!mp.count(dist[w])) mp[dist[w]] = len[w];
else mp[dist[w]] = min(mp[dist[w]], len[w]);
}
}
}
void decomp(int u, int par) {
calc(u, -1);
int c = centroid(u, par, sub[u] / 2);
solve(c); vis[c] = 1;
for(int v : adj[c]) if(!vis[v]) decomp(v, c);
}
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]].push_back(H[i][1]);
adj[H[i][1]].push_back(H[i][0]);
cost[H[i][0]].push_back(L[i]);
cost[H[i][1]].push_back(L[i]);
}
decomp(0, -1);
if(ans == 1e9) ans = -1;
return ans;
}
