样例通过，提交 60 pts

#include <bits/stdc++.h>

using namespace std;

struct node {
	int u;
	int v;
} e[600015];

int n, m;
int pnt;
int head[600015];
int dp[600015];

void AddEdge (int u, int v) {
	e[++pnt].u = v;
	e[pnt].v = head[u];
	head[u] = pnt;
}

void dfs (int cur, int fa) {
	bool isleaf = true;
	int cnt = 0;
	int Max = 0, Semax = 0;
	for (int p = head[cur]; p; p = e[p].v) {
		if (e[p].u == fa) continue;
		dfs(e[p].u, cur);
		isleaf = false;
		dp[cur] = max(dp[cur], dp[e[p].u]);
		cnt++;
		if (dp[e[p].u] > Max) {
			Semax = Max;
			Max = dp[e[p].u];
		} else if (dp[e[p].u] > Semax) Semax = dp[e[p].u];
	}
	if (isleaf) 
		dp[cur] = 1;
	else
		dp[cur] += cnt;
	if (isleaf)
		pnt = max(pnt, 1);
	else if (cnt == 1)
		pnt = max(pnt, Max + 1);
	else
		pnt = max(pnt, Max + Semax + cnt - 1); 
}

int main () {
	scanf("%d%d", &n, &m);
	for (int i = 1, u, v; i < n; i++) {
		scanf("%d%d", &u, &v);
		AddEdge(u, v);
		AddEdge(v, u);
	}
	pnt = 0;
	dfs(1, -1); 
	printf("%d", pnt);
	return 0;
}