#include <iostream>
#include <map>
#include <vector>
using namespace std;
const int MAXN = 2e5 + 10;
const int oo = 1e9;
typedef pair < int, int > II;
int sz[MAXN], depth[MAXN];
long long sum[MAXN];
vector < II > adj[MAXN];
vector < int > vec[MAXN];
map < long long, int > path;
int h[MAXN][2], l[MAXN];
int n, k;
int ans = oo;
void dfs(int u, int p) {
sz[u] = 1;
for(int i = 0; i < adj[u].size(); i++) {
II cur = adj[u][i];
int v = cur.first, w = cur.second;
if (v == p)
continue;
depth[v] = depth[u] + 1;
sum[v] = sum[u] + w;
dfs(v, u);
sz[u] += sz[v];
}
}
void add(int u, int k) {
long long sumNow = sum[u];
int & temp = path[sumNow];
if (k == -1) {
path[sumNow] = 0;
} else {
int tmp = temp;
if (tmp == 0)
tmp = oo;
temp = min(tmp, depth[u]);
}
}
void sack(int u, int p, int keep) {
int szMax = -1, bigChild = -1;
for(int i = 0; i < adj[u].size(); i++) {
int v = adj[u][i].first;
if (v == p)
continue;
if (sz[v] > szMax) {
sz[v] = szMax;
bigChild = v;
}
}
for(int i = 0; i < adj[u].size(); i++) {
int v = adj[u][i].first;
if (v != bigChild && v != p)
sack(v, u, 0);
}
if (bigChild != -1) {
sack(bigChild, u, 1);
swap(vec[u], vec[bigChild]);
}
vec[u].push_back(u);
add(u, 1);
if (path[k + sum[u]])
ans = min(ans, path[k + sum[u]] - depth[u]);
for(int i = 0; i < adj[u].size(); i++) {
int v = adj[u][i].first;
if (v == p || v == bigChild)
continue;
for(int j = 0; j < vec[v].size(); j++) {
int x = vec[v][j];
long long need = k + sum[u] * 2 - sum[x];
if (path[need])
ans = min(ans, path[need] + depth[x] - 2 * depth[u]);
}
for(int j = 0; j < vec[v].size(); j++) {
int x = vec[v][j];
add(x, 1);
vec[u].push_back(x);
}
}
if (!keep) {
for(int i = 0; i < vec[u].size(); i++) {
int v = vec[u][i];
add(v, -1);
}
}
}
int solve() {
dfs(0, -1);
sack(0, -1, 1);
return (ans == oo ? -1 : ans);
}
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 u = H[i][0], v = H[i][1], w = L[i];
adj[u].push_back(II(v, w));
adj[v].push_back(II(u, w));
}
return solve();
}
