#include <bits/stdc++.h>
#include <race.h>
using namespace std;
typedef long long int ll;
const int M = 2e5 + 100;
struct Edge{
int next;
ll len;
};
int n, sub[M];
ll val[M], depth[M], k, best;
vector<Edge> g[M];
map<ll, pair<bool, int>> a[M];
void dfs(int v, int p){
int mx = 0, big = -1;
for(Edge e: g[v]){
if(e.next != p && sub[e.next] > mx){
mx = sub[e.next];
big = e.next;
}
}
for(Edge e: g[v]){
if(e.next != p && e.next != big)
dfs(e.next, v);
}
if(big > -1)
dfs(big, v), a[v].swap(a[big]);
a[v][val[v]].first = 1;
a[v][val[v]].second = depth[v];
for(Edge e: g[v]){
if(e.next != big && e.next != p){
for(auto p: a[e.next]){
if(a[v][k - p.first + 2 * val[v]].first == 1){
best = min(best, p.second.second + a[v][k - p.first + 2 * val[v]].second - depth[v] * 2);
}
if(a[v][p.first].first == 0){
a[v][p.first] = {1, p.second.second};
}else a[v][p.first].second = min(a[v][p.first].second, p.second.second);
}
}
}
if(a[v][k + val[v]].first == 1){
best = min(best, a[v][k + val[v]].second - depth[v]);
}
}
void init(int v, int p, ll d, int dd){
sub[v] = 1;
val[v] = d;
depth[v] = dd;
for(Edge e: g[v]){
if(e.next != p){
init(e.next, v, d + e.len, dd + 1);
sub[v] += sub[e.next];
}
}
}
int best_path(int N, int K, int H[][2], int L[])
{
best = N + 1;
n = N;
k = K;
Edge e;
for(int i = 0; i < n-1; ++i){
e.next = H[i][1];
e.len = L[i];
g[H[i][0]].push_back(e);
e.next = H[i][0];
g[H[i][1]].push_back(e);
}
init(0, 0, 0, 0);
dfs(0, 0);
return (best == N+1 ? -1 : best);
}
