#include<cstdio>
#include<algorithm>
using namespace std;
int v[200010],first[100010],next[200010],e;
void AddEdge(int U,int V){
	v[++e]=V;
	next[e]=first[U];
	first[U]=e;
}
int n,m;
int S1;
bool vis[100010],vis2[100010];
int maxD;
void dfs(int U,int d){
	vis[U]=1;
	if(d>maxD){
		maxD=d;
		S1=U;
	}
	for(int i=first[U];i;i=next[i]){
		if(!vis[v[i]]){
			dfs(v[i],d+1);
		}
	}
}
void df2(int U,int d){
	vis2[U]=1;
	maxD=max(maxD,d);
	for(int i=first[U];i;i=next[i]){
		if(!vis2[v[i]]){
			df2(v[i],d+1);
		}
	}
}
int b[100010],be;
int main(){
//	freopen("a.in","r",stdin);
	int x,y;
	scanf("%d%d",&n,&m);
	if(n==1){
		puts("0");
		return 0;
	}
	if(n==2){
		puts("1");
		return 0;
	}
	for(int i=1;i<=m;++i){
		scanf("%d%d",&x,&y);
		++x;
		++y;
		AddEdge(x,y);
		AddEdge(y,x);
	}
	for(int i=1;i<=n;++i){
		if(!vis[i]){
			maxD=0;
			dfs(i,0);
			maxD=0;
			df2(S1,0);
			b[++be]=maxD;
		}
	}
	sort(b+1,b+be+1);
	int tmp=max(2,max(b[be]/2+b[be]%2+b[be-1]/2+b[be-1]%2+(be!=1),b[be]));
	if(be>=3){
		tmp=max(tmp,b[be-2]/2+b[be-2]%2+b[be-1]/2+b[be-1]%2+2);
	}
	printf("%d\n",tmp);
	return 0;
}