#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#define MAX_V 200005
using namespace std;
int n,ans,ans1[MAX_V],ans2[MAX_V],c[MAX_V][6],cnum[MAX_V],id[MAX_V],dis[MAX_V][6],m;
vector<int> G[MAX_V];
vector<int> rG[MAX_V];
vector<int> vs;
bool used[MAX_V];
int cmp[MAX_V];
void add_edge(int from,int to)
{
	G[from].push_back(to);
	rG[to].push_back(from);
}
void dfs(int v)
{
	used[v]=1;
	for(int i=0;i<G[v].size();++i)
	{
		if(!used[G[v][i]])
			dfs(G[v][i]);
	}
	vs.push_back(v);
}
void rdfs(int v,int k)
{
	used[v]=1;
	cmp[v]=k;
	for(int i=0;i<rG[v].size();++i)
	{
		if(!used[rG[v][i]])
		rdfs(rG[v][i],k);
	}
}
int scc()
{
	memset(used,0,sizeof used);
	vs.clear();
	for(int v=1;v<=n;v++)
	{
		if(!used[v]) dfs(v);
	}
	memset(used,0,sizeof used);
	int k=1;
	for(int i=vs.size()-1;i>=0;--i)
	{
		if(!used[vs[i]]) 
		rdfs(vs[i],k++);
	}
	return k;
}
void cal(int now,int nowid,int f,int nowdis)
{
	used[now]=1;
	dis[now][nowid]=max(dis[now][nowid],nowdis);
	for(int i=0;i<G[now].size();++i)
	if(cmp[G[now][i]]==f&&!used[G[now][i]])
	{
		cal(G[now][i],nowid,f,nowdis+1);
	}
	used[now]=0;
	return;
}
int get2(int now);
int get1(int now)
{
	if(ans1[now]!=-1) return ans1[now];
	int nowans=1;
	for(int i=0;i<rG[now].size();i++)
	if(cmp[rG[now][i]]!=cmp[now])
	{
		nowans=max(nowans,get2(rG[now][i])+1);
	}
	return ans1[now]=nowans;
}
int get2(int now)
{
	if(ans2[now]!=-1) return ans2[now];
	int nowans=-1;
	for(int i=1;i<=cnum[cmp[now]];++i)
	{
		nowans=max(nowans,get1(c[cmp[now]][i])+dis[now][i]);
	}
	return ans2[now]=nowans;
}
int main()
{
	scanf("%d%d",&n,&m);
	int u,v;
	for(int i=1;i<=m;++i)
	{
		scanf("%d%d",&u,&v);
		add_edge(u,v);
	}
	scc();
	memset(used,0,sizeof used);
	for(int i=1;i<=n;++i)
	{
		ans1[i]=ans2[i]=-1;
		c[cmp[i]][++cnum[cmp[i]]]=i;
		id[i]=cnum[cmp[i]];
		cal(i,id[i],cmp[i],0);
	}
	for(int i=1;i<=n;++i)
	{
		ans=max(ans,get2(i));
	}
	cout<<ans;
	return 0;
}