//一時保存

#include "bits/stdc++.h"
using namespace std;
#define int long long
#define endl '\n'
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
	int n,m;
	cin>>n>>m;
	vector<int> G[n];
	vector<int> cnt(n,0);
	for(int i=0;i<m;i++){
		int a,b;
		cin>>a>>b;
		a--;
		b--;
		cnt[b]++;
		G[a].push_back(b);
	}

	queue<int> q;
	for(int i=0;i<n;i++){
		if(cnt[i]==0)q.push(i);
	}

	vector<int> dp(n,0);
	while(!q.empty()){
		int np=q.front();
		q.pop();
		for(int i=0;i<G[np].size();i++){
			cnt[G[np][i]]--;
			if(cnt[G[np][i]]==0){
				q.push(G[np][i]);
			}
			dp[G[np][i]]=max(dp[G[np][i]],dp[np]+1);
		}
	}
	int ans=0;
	for(int i=0;i<n;i++)ans=max(ans,dp[i]);
	cout<<ans<<endl;
}

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