思路：树的直径+优先队列

#include <bits/stdc++.h>
using namespace std;
#define int long long
struct sss {
	int w, x;
	bool operator<(const sss& a)const {
		return w < a.w;
	}
};
bool st[200010];
int h[200010], e[300010], ne[300010], idx, deep[200010], maxi, maxj, d[200010], midroot;
void add(int a, int b) {
	ne[idx] = h[a], e[idx] = b, h[a] = idx++;
	return;
}
void bfs() {
	queue<int>q;
	q.push(1);
	deep[1] = 1;
	while (q.size()) {
		int u = q.front();
		q.pop();
		maxi = u;
		for (int i = h[u]; i != -1; i = ne[i]) {
			int j = e[i];
			if (deep[j] <= deep[u])continue;
			deep[j] = deep[u] + 1;
			q.push(j);
		}
	}
	return;
}
void bfs2(int sta) {
	queue<int>q;
	q.push(sta);
	d[sta] = 1;
	while (q.size()) {
		int u = q.front();
		q.pop();
		maxj = u;
		for (int i = h[u]; i != -1; i = ne[i]) {
			int j = e[i];
			if (d[j] <= d[u])continue;
			d[j] = d[u] + 1;
			q.push(j);
		}
	}
	return;
}
bool dfs(int u, int fa, int dist) {
	if (u == maxj)return true;
	for (int i = h[u]; i != -1; i = ne[i]) {
		int j = e[i];
		if (j == fa)continue;
		if (dfs(j, u, dist + 1)) {
			if (dist == d[maxj] >> 1) midroot = u;
			return true;
		}
	}
	return false;
}
int dfs2(int u, int fa) {
	int w = 0;
	for (int i = h[u]; i != -1; i = ne[i]) {
		int j = e[i];
		if (j == fa)continue;
		w = max(w, dfs2(j, u));
	}
	return w + 1;
}
signed main() {
	memset(d, 0x3f, sizeof d);
	memset(deep, 0x3f, sizeof deep);
	int n, k;
	cin >> n >> k;
	memset(h, -1, sizeof h);
	for (int i = 1; i < n; i++) {
		int u, v;
		cin >> u >> v;
		add(u, v);
		add(v, u);
	}
	bfs();
	bfs2(maxi);
	dfs(maxi, 0, 0);
	priority_queue<sss>q;
	int maxx = 0;
	st[midroot] = true;
	for (int i = h[midroot]; i != -1; i = ne[i]) {
		int j = e[i];
		q.push({ dfs2(j, midroot), j });
		st[j] = true;
	}
	for (int kk = 1; kk < k; kk++) {
		sss now = q.top();
		q.pop();
		for (int i = h[now.x]; i != -1; i = ne[i]) {
			int j = e[i];
			if (st[j])continue;
			st[j] = true;
			q.push({ now.w - 1, j });
		}
	}
	if (!q.size()) cout << 0;
	else cout << q.top().w;
	return 0;
}