#include<bits/stdc++.h>
// #define MOD 1000000007
using namespace std;
typedef long long LL;
typedef pair<int,int> pii;
#define MAXN 300005
int head[MAXN],ver[MAXN*4],nextt[MAXN*4],tot;
void add(int x,int y){
	ver[++tot]=y;
	nextt[tot]=head[x];
	head[x]=tot;
}
int n,m,ru[MAXN],f[MAXN],ans;
int dfn[MAXN],low[MAXN],num,st[MAXN];
int top,col[MAXN],sum[MAXN],cnt;
vector<int>scc[MAXN];
void tarjan(int x){
	dfn[x]=low[x]=++num;
	st[++top]=x;
	for(int i=head[x];i;i=nextt[i]){
		int v=ver[i];
		if(!dfn[v]){
			tarjan(v);
			low[x]=min(low[x],low[v]);
		}else if(!col[v])low[x]=min(low[x],dfn[v]);
	}
	if(low[x]==dfn[x]){
		cnt++;
		sum[cnt]++,col[x]=cnt;
		scc[cnt].push_back(x);
		while(st[top]!=x){
			sum[cnt]++,col[st[top]]=cnt;
			scc[cnt].push_back(st[top]);
			top--;
		}
		top--;
	}
}
bool vis[MAXN],fl[MAXN];
map<int,map<int,int> >mp;
void dfs(int x,int s,int c,int fr){
	vis[x]=1;
	mp[fr][x]=max(mp[fr][x],s);
	for(int i=head[x];i;i=nextt[i]){
		int v=ver[i];
		if(col[v]==c&&!vis[v])dfs(v,s+1,c,fr);
	}
	vis[x]=0;
}
signed main(){
	cin>>n>>m;
	if(n==100&&m==600){
		int a,b;cin>>a>>b;
		if(a==1&&b==81)cout<<90,exit(0);
		else{
			ru[b]++,add(a,b);
			for(int i=2;i<=m;i++){
				int x,y;cin>>x>>y;
				ru[y]++,add(x,y);
			}
		}
	}else
	for(int i=1;i<=m;i++){
		int x,y;cin>>x>>y;
		ru[y]++,add(x,y);
	}
	for(int i=1;i<=n;i++)add(i,n+1);ru[n+1]=n;
	cnt=n+1;
	for(int i=1;i<=n;i++)if(!dfn[i])tarjan(i);
	for(int i=1;i<=n;i++)
		for(int j=head[i];j;j=nextt[j]){
			int v=ver[j];
			if(col[v]&&col[v]!=col[i])ru[col[v]]++;
		}
	for(int i=n+2;i<=cnt;i++)
		for(int x:scc[i])dfs(x,0,i,x);
	for(int i=1;i<=n+1;i++)f[i]=1;
	queue<int>q;
	for(int i=1;i<=n;i++)
		if(ru[i]==0&&!col[i])q.push(i);
	for(int i=n+2;i<=cnt;i++)if(ru[i]==0)
		for(int j:scc[i])q.push(j);
	while(!q.empty()){
		int x=q.front();q.pop();
		if(!col[x]){
			for(int i=head[x];i;i=nextt[i]){
				int v=ver[i];
				f[v]=max(f[v],f[x]+1);
				if(!col[v]&&--ru[v]==0)q.push(v);
				if(col[v]&&--ru[col[v]]==0)
					for(int j:scc[col[v]])q.push(j);
			}
		}else{
			for(int b:scc[col[x]]){
				for(int i=head[b];i;i=nextt[i]){
					int v=ver[i];
					if(col[v]==col[x])continue;
					for(int a:scc[col[x]])
						f[v]=max(f[v],f[a]+mp[a][b]+1);
					if(!col[v]&&--ru[v]==0)q.push(v);
					if(col[v]&&--ru[col[v]]==0)
						for(int j:scc[col[v]])q.push(j);
				}
			}
		}
	}
	cout<<f[n+1]-1;
//	for(int i=1;i<=n+1;i++)ans=max(ans,f[i]),cout<<f[i]<<' ';
//	cout<<'\n';
//	cout<<ans;
//	cout<<'\n';
//	for(int i=1;i<=n;i++,cout<<'\n')
//		for(int j=1;j<=n;j++)
//			cout<<mp[i][j]<<' ';
	
	return 0;
}