#include <bits/stdc++.h>
using namespace std;
const long long maxn = 2e5 + 12, mod = 998244353, inf = 1e9 + 12 ;
long long n, k, ans = maxn, h[maxn], sz[maxn], cost[maxn], U[maxn], V[maxn], dist[maxn];
int H2[maxn][2], L2[maxn];
vector <int> conn[maxn];
map <int, int> mp[maxn];
bitset <maxn> marked;
void dfs_set(int u){
marked[u] = 1;
sz[u] = 1;
for(int e: conn[u]){
int v = u ^ U[e] ^ V[e];
if(!marked[v]){
h[v] = h[u] + 1;
dist[v] = dist[u] + cost[e];
dfs_set(v);
sz[u] += sz[v];
}
}
}
void dfs(int u, int p = -1){
marked[u] = 1;
for(int e: conn[u]){
int v = u ^ U[e] ^ V[e];
if(!marked[v]){
dfs(v, u);
if(mp[v].find(k + dist[u]) != mp[v].end())
ans = min(ans, mp[v][k + dist[u]] - h[u]);
}
}
pair<long long, int> mx = make_pair(0, -1);
for(int e: conn[u]){
int v = u ^ U[e] ^ V[e];
if(v != p)
mx = max(mx, make_pair(sz[v], v));
}
int f = mx.second;
if(f != -1){
swap(mp[f], mp[u]);
for(int e: conn[u]){
int v = u ^ U[e] ^ V[e];
if(v != p && v != f){
for(auto len: mp[v]){
if(mp[u].find(k + dist[u] * 2 - len.first) != mp[u].end())
ans = min(ans, mp[u][k + dist[u] * 2 - len.first] + len.second - 2 * h[u]);
}
for(auto len: mp[v]){
int x = mp[u][len.first];
if(x == 0)
mp[u][len.first] = len.second;
else
mp[u][len.first] = min(x, len.second);
}
}
}
}
long long x = mp[u][dist[u]];
if(x == 0)
mp[u][dist[u]] = h[u];
else
mp[u][dist[u]] = min(x, h[u]);
}
int best_path(int N, int K, int H[][2], int L[]){
n = N;
k = K;
for(int i = 0; i < n - 1; i++){
cost[i] = L[i];
U[i] = H[i][0];
V[i] = H[i][1];
conn[U[i]].push_back(i);
conn[V[i]].push_back(i);
}
// cout << n << ' ' << k << endl;
dfs_set(1);
marked = 0;
dfs(1);
if(ans == maxn)
ans = -1;
return ans;
}
/*
int main(){
int N, K;
cin >> N >> K;
for(int i = 0; i < N - 1; i++)
cin >> H2[i][0] >> H2[i][0];
for(int i = 0; i < N - 1; i++)
cin >> L2[i];
cout << best_path(N, K, H2, L2) << endl;
}
*/
