#include <bits/stdc++.h>
#define ll long long
#define pll pair<ll, ll>
#define pii pair<int, int>
#define io(x) if (fopen(x".inp","r")) {freopen(x".inp","r",stdin),freopen(x".out","w",stdout);}
#define mem(c, x) memset(c, x, sizeof(c))
#define all(c) c.begin(), c.end()
#define bit(i,j) ((i >> j) & 1)
#define pb push_back
#define se second
#define fi first
#define el '\n'
using namespace std;
template<class T> bool maximize(T &a, const T &b) { return (a < b ? a = b, 1 : 0); }
template<class T> bool minimize(T &a, const T &b) { return (a > b ? a = b, 1 : 0); }
int dx[8] = {0, 1, 0,-1, 1, 1,-1,-1};
int dy[8] = {1, 0,-1, 0, 1,-1,-1, 1};
const int maxn = 2e5 + 9;
const int Inf = 2e9 + 7;
const ll Infll = 1e18 + 9;
const ll Mod = 1e9 + 7;
/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
int n, k;
vector<pii> adj[maxn]; ll sum[maxn];
int h[maxn], sz[maxn], res = 0x3f3f3f3f;
map<ll, int> mp;
vector<int> vec[maxn];
void dfs(int u, int par) {
sz[u] = 1;
for (auto x : adj[u]) {
int v = x.fi, w = x.se;
if (v == par) continue;
sum[v] = sum[u] + w;
h[v] = h[u] + 1;
dfs(v, u);
sz[u] += sz[v];
}
}
void sack(int u, int par, bool keep) {
int hevNode = 0;
for (auto v : adj[u]) {
if (v.fi == par) continue;
if (!hevNode || sz[hevNode] < sz[v.fi]) {
hevNode = v.fi;
}
}
for (auto v : adj[u]) {
if (v.fi == par || v.fi == hevNode) continue;
sack(v.fi, u, 0);
}
if (hevNode) {
sack(hevNode, u, 1);
swap(vec[hevNode], vec[u]);
}
vec[u].pb(u);
if (mp.find(sum[u]) == mp.end()) mp[sum[u]] = 0x3f3f3f3f;
minimize(mp[sum[u]], h[u]);
if (mp.find(k + sum[u]) != mp.end())
minimize(res, mp[k + sum[u]] - h[u]);
for (auto z : adj[u]) {
int v = z.fi, w = z.se;
if (v == par || v == hevNode) continue;
for (int x : vec[v]) {
if (mp.find(k + 2ll*sum[u] - sum[x]) != mp.end())
minimize(res, h[x] - 2*h[u] + mp[k + 2ll*sum[u] - sum[x]]);
}
for (int x : vec[v]) {
vec[u].pb(x);
if (mp.find(sum[x]) == mp.end()) mp[sum[x]] = 0x3f3f3f3f;
minimize(mp[sum[x]], h[x]);
}
}
if (!keep) {
mp.clear();
}
}
int best_path(int N, int K, int H[][2], int L[]) {
n = N; k = K;
for (int i=0; i<n-1; i++) {
int x = H[i][0] + 1;
int y = H[i][1] + 1;
int w = L[i];
adj[x].pb({y, w});
adj[y].pb({x, w});
}
dfs(1, 1);
sack(1, 1, 1);
if (res == 0x3f3f3f3f) return -1;
return res;
}
