#include <algorithm> #include <cstring> #include <iostream> #include <vector> using namespace std; struct edge { int v, w; edge(int v = 0, int w = 0) { this->v = v; this->w = w; } }; const int maxn = 300000 + 5; vector<edge> e[maxn]; int dep[maxn], f[maxn], c; int dia[maxn], cnt, pres[maxn], posts[maxn]; bool vis[maxn]; int n, s; void dfs(int u, int fa) { f[u] = fa; for (auto ed : e[u]) { if (ed.v == fa || vis[ed.v]) continue; dep[ed.v] = dep[u] + ed.w; if (dep[ed.v] > dep[c]) c = ed.v; dfs(ed.v, u); } } void get_diameter() { dfs(1, 0); dep[c] = 0; dfs(c, 0); for (int u = c; u; u = f[u]) { dia[++cnt] = u; pres[cnt] = dep[u]; } reverse(dia + 1, dia + cnt + 1); reverse(pres + 1, pres + cnt + 1); for (int i = cnt; i > 0; i--) posts[i] = pres[cnt] - pres[i]; } namespace sub1 { void solve() { int minecc = 1 << 30; for (int i = 1; i <= cnt; i++) for (int j = i; j <= cnt; j++) { if (pres[j] - pres[i] <= s) { memset(vis, 0, sizeof(vis)); for (int k = i; k <= j; k++) vis[dia[k]] = true; int ecc = 0; for (int k = i; k <= j; k++) { dep[dia[k]] = 0, c = 0; dfs(dia[k], 0); ecc = max(ecc, dep[c]); } minecc = min(minecc, ecc); } } cout << minecc << endl; } } // namespace sub1 namespace sub2 { void solve() { int minecc = 1 << 30; int l = 1, r = 1; for (; l <= cnt; l++) { while (r <= cnt && pres[r + 1] - pres[l] <= s) r++; memset(vis, 0, sizeof(vis)); for (int k = l; k <= r; k++) vis[dia[k]] = 1; int ecc = 0; for (int k = l; k <= r; k++) { dep[dia[k]] = 0, c = 0; dfs(dia[k], 0); ecc = max(ecc, dep[c]); } minecc = min(minecc, ecc); } cout << minecc << endl; } } // namespace sub2 namespace sub3 { int maxd[maxn]; bool check(int ecc) { int l = 1, r = cnt; while (l < cnt && pres[l + 1] <= ecc) l++; while (r > l && posts[r - 1] <= ecc) r--; if (pres[r] - pres[l] > s) return false; int d = 0; for (int i = l + 1; i < r; i++) d = max(d, maxd[i]); return d <= ecc; } void solve() { for (int i = 1; i <= cnt; i++) vis[dia[i]] = true; for (int i = 1; i <= cnt; i++) { dep[dia[i]] = 0, c = 0; dfs(dia[i], 0); maxd[i] = dep[c]; } int l = 0, r = 1 << 30, ans = 0; while (l <= r) { int mid = (l + r) >> 1; if (check(mid)) ans = mid, r = mid - 1; else l = mid + 1; } cout << ans << endl; } } // namespace sub3 namespace sub4 { void solve() { for (int i = 1; i <= cnt; i++) vis[dia[i]] = true; int maxd = 0; for (int i = 1; i <= cnt; i++) { dep[dia[i]] = 0, c = 0; dfs(dia[i], 0); maxd = max(dep[c], maxd); } int l = 1, r = 1; int minecc = 1 << 30; for (; l <= cnt; l++) { while (r <= cnt && pres[r + 1] - pres[l] <= s) r++; minecc = min(max(maxd, max(pres[l], posts[r])), minecc); } cout << minecc << endl; } } // namespace sub4 int main() { ios::sync_with_stdio(false); cin >> n >> s; for (int i = 1; i < n; i++) { int u, v, w; cin >> u >> v >> w; e[u].emplace_back(v, w); e[v].emplace_back(u, w); } get_diameter(); sub4::solve(); return 0; }