#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
using namespace std;
long long n, m;
vector<pair<int, int>> graph[200009];
long long prefix[200009];
int node2disc[200009];
int disc2node[400009];
int disc2dept[400009];
int cnt = 0;
void dfs1(int a, int par, int dept) {
node2disc[a] = ++cnt;
disc2node[cnt] = a;
disc2dept[cnt] = dept;
for (pair<int, int> x : graph[a]) {
if (x.first == par) continue;
prefix[x.first] = prefix[a] + x.second;
dfs1(x.first, a, dept + 1);
disc2node[cnt] = a;
disc2dept[cnt] = dept;
}
cnt++;
}
pair<int, int> rmmq[400009][19];
void build_rmq() {
for (int i = 1; i < cnt; i++) {
rmmq[i][0] = {disc2dept[i], i};
for (int j = 0; (1 << (j + 1)) <= i; j++) {
rmmq[i][j + 1] = min(rmmq[i - (1 << j)][j], rmmq[i][j]);
}
}
}
pair<int, int> get_rmq(int u, int v) {
if (u == v) return rmmq[u][0];
if (u > v) swap(u, v);
int lg = 31 - __builtin_clz(v - u + 1);
return min(
rmmq[u + (1 << lg) - 1][lg],
rmmq[v][lg]
);
}
int get_lca(int u, int v) {
return disc2node[get_rmq(node2disc[u], node2disc[v]).second];
}
int calc_dist(int u, int v) {
return prefix[u] + prefix[v] - (2 * prefix[get_lca(u, v)]);
}
int calc_cnt_dist(int u, int v) {
return disc2dept[node2disc[u]] + disc2dept[node2disc[v]] - (2 * get_rmq(node2disc[u], node2disc[v]).first);
}
int centroid_parent[200009] = {};
bool blocked[200009] = {};
int get_tree_size(int a, int par) {
int sum = 1;
for (pair<int, int> x : graph[a]) {
if (x.first == par|| blocked[x.first]) continue;
sum += get_tree_size(x.first, a);
}
return sum;
}
int get_centroid(int a, int par, int size) {
int sum = 1;
for (pair<int, int> x : graph[a]) {
if (x.first == par || blocked[x.first]) continue;
int ehe = get_centroid(x.first, a, size);
if (ehe < 0) return ehe;
sum += ehe;
}
if (sum > size / 2) return -a;
return sum;
}
int build_centroid_tree(int a) {
int centroid = -get_centroid(a, a, get_tree_size(a, a));
// cout << a << " = " << centroid << " " << get_tree_size(a, a) << endl;
blocked[centroid] = 1;
for (pair<int, int> x : graph[centroid]) {
if (blocked[x.first]) continue;
int ch = build_centroid_tree(x.first);
centroid_parent[ch] = centroid;
// cout << ch << " " << centroid << endl;
}
return centroid;
}
map<long long, int> dist_to_child[200009];
int ans = 1 << 30;
void query(int a) {
int gay = a;
while (gay > 0) {
int dist = calc_dist(gay, a);
int cnt = calc_cnt_dist(gay, a);
if (dist_to_child[gay].find(m - dist) != dist_to_child[gay].end()) {
ans = min(ans, dist_to_child[gay][m - dist] + cnt);
}
if (dist_to_child[gay].find(dist) == dist_to_child[gay].end()) {
dist_to_child[gay][dist] = cnt;
} else {
dist_to_child[gay][dist] = min(dist_to_child[gay][dist], cnt);
}
gay = centroid_parent[gay];
}
}
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie();
cin >> n >> m;
for (int i = 1; i < n; i++) {
int x, y, z;
cin >> x >> y >> z;
x++;
y++;
graph[x].push_back({y, z});
graph[y].push_back({x, z});
}
dfs1(1, 1, 1);
build_rmq();
build_centroid_tree(1);
for (int i = 1; i <= n; i++) {
query(i);
}
if (ans == 1 << 30) {
cout << -1;
return 0;
}
cout << ans;
return 0;
}
