/*
                       _oo0oo_
                      o8888888o
                      88" . "88
                      (| -_- |)
                      0\  =  /0
                    ___/`---'\___
                  .' \\|     |// '.
                 / \\|||  :  |||// \
                / _||||| -:- |||||- \
               |   | \\\  -  /// |   |
               | \_|  ''\---/''  |_/ |
               \  .-\__  '-'  ___/-. /
             ___'. .'  /--.--\  `. .'___
          ."" '<  `.___\_<|>_/___.' >' "".
         | | :  `- \`.;`\ _ /`;.`/ - ` : | |
         \  \ `_.   \_ __\ /__ _/   .-` /  /
     =====`-.____`.___ \_____/___.-`___.-'=====
                       `=---='
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  Vikash Kumar @ Codechef/codeforces 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/   
#include<bits/stdc++.h>
 
#define ll                    long long int
#define ll_MAX		      	  LLONG_MAX
#define ll_MIN		          LLONG_MIN
#define loop(i,a,b)           for(int i=(int)a;i<(int)b;++i)
#define rloop(i,a,b)          for(int i=(int)a;i<=(int)b;++i)
#define loopl(i,a,b)          for(ll i=(ll)a;i<(ll)b;++i)
#define loopr(i,a,b)          for(int i=(int)a;i>=(int)b;--i)
#define count_1(n)            __builtin_popcountll(n)
#define pb                    push_back
#define eb                    emplace_back
#define ab(a)                 (a<0)?(-1*a):a
#define mset(a,b,c)           loop(i,0,b) a[i]=c
#define F                     first
#define S                     second
//#define mp                    make_pair
#define clr(x)                x.clear()
#define MOD                   1000000007
#define MAX                   1e9
#define MIN                   -1e9
#define itoc(c)               ((char)(((int)'0')+c))
#define vi                    vector<int>
#define vvi                   vector<vi>
#define pll                   pair<ll,ll>
#define pii                   pair<int,int>
#define all(p)                p.begin(),p.end()
//#define max(x,y)              (x>y)?x:y
//#define min(x,y)              (x<y)?x:y
#define mid(s,e)              (s+(e-s)/2)
#define mini                  INT_MIN
const int MX=10010896;
const int lmt=3164;
const int N=10000001;
int flag[MX>>6];
#define ifc(i)                (flag[i>>6]&(1<<((i>>1)&31)))
#define isc(i)                (flag[i>>6]|=(1<<((i>>1)&31)))
#define chk(n)                (n==2||(n>2&&(n&1)&&!ifc(n)))
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
using namespace std;
ll extgcd(ll a,ll b,ll& x,ll& y){if(b==0){x=1;y=0;return a;}else{int g=extgcd(b,a%b,y,x);y-=a/b*x;return g;}}
ll modpow(ll a,ll b) {ll res=1;a%=MOD;for(;b;b>>=1){if(b&1)res=res*a%MOD;a=a*a%MOD;}return res;}
void sieve(){ int i,j,k; for(i=3;i<lmt;i+=2) if(!ifc(i)) for(j=i*i,k=i<<1;j<MX;j+=k) isc(j);}


void prntdp(int n,int *dp){
	for(int i=0;i<n;i++)
		cout<<"i{"<<i<<"}: "<<dp[i]<<" ";
	cout<<endl;
}

void dfs(int u,vector<int> *adj,bool *visited,int *dp){
	visited[u]=true;
	for(auto v:adj[u]){
		if(!visited[v]){
			dfs(v,adj,visited,dp);
		}
		dp[u]=max(dp[u],1+dp[v]);
	}
	//cout<<"u{"<<u<<"}: "<<dp[u]<<endl;
}

inline void solve(){
    /*_Start_*/
	int n,m;cin>>n>>m;
	vector<int> adj[n];
	for(int i=0;i<m;i++){
		int u,v;cin>>u>>v;
		u--;v--;
		adj[u].push_back(v);
	}
	int dp[n];
	bool visited[n];
	for(int i=0;i<n;i++){
		visited[i]=false;
		dp[i]=0;
	}
	
	for(int i=0;i<n;i++){
		if(!visited[i])
			dfs(i,adj,visited,dp);
	}
	int ans=0;
	for(int i=0;i<n;i++){
		ans=max( ans,dp[i] );
	}
	//prntdp(n,dp);
	cout<<ans<<endl;
	

    /*_End_*/
   
}

int main()
{
	cout<<fixed<<setprecision(12);
	//freopen("input.txt", "r", stdin);
	fastio;ll  t=1,tc=1;
	//cin>>t;
	while(t--){solve();}return 0;
} 

Measured memory_bytes: 9600