// Slower C++ solution:
#include<iostream>

#include<iomanip>

#include<cmath>

#include<string>

#include<cctype>

#include<algorithm>

#include<set>

#include<map>

#include<stack>

#include<queue>

#include<cstring>

//#include<windows.h>

//#include<time.h>

#define pb push_back

#define ppb pop_back

#define fi first

#define se second

#define mer make_pair

#define mme memset

using namespace std;



const double pie1=3.1415926;//π1

const double pie2=3.14159265358979;//π2

const double pie3=3.14159265358979323846;//π3

const int vbi=999999999;

const int vli=-vbi;

const long long vbl=999999999999999999;

const long long vll=-vbl;



typedef long long ll;

typedef pair<int,int> pii;

typedef pair<ll,ll> pll;

typedef pair<pii,int> piii;

typedef pair<pii,pii> pi4;

typedef pair<pll,ll> plll;

typedef pair<int,pii> ipii;

typedef pair<ll,pll> lpll;

typedef pair<pll,pll> pl4;

typedef vector<int> vi;

typedef vector<ll> vl;

typedef vector<pii> vpi;

typedef vector<pll> vpl;

typedef set<int> sti;

typedef set<ll> stl;

typedef set<pii> spi;

typedef set<pll> spl;

priority_queue<int> pqi;

priority_queue<ll> pql;

queue<int> qi;

queue<ll> ql;



/*void bin_v(){

	char bin[1000];

	gets(bin);

}*/



bool pss(int a){

	for(int i=2;i<=sqrt(a);i++)

		if(a%i==0)

			return false;

	return true;

}

/////////////////////////////////////////////////////////////////////////////////////////

int n,m;

vector<int> v[100010];

int d[100010];

int dp[100010];



int main(){

	//my name is Veex_WCY

	ios::sync_with_stdio(false);

	cin>>n>>m;

	for(int i=1;i<=m;i++){

		int a,b;

		cin>>a>>b;

		v[a].pb(b);

		d[b]++;

	}

	stack<int> st;

	for(int i=1;i<=n;i++)

		if(d[i]==0)

			st.push(i);

	while(!st.empty()){

		int x=st.top();

		st.pop();

		for(int i=0;i<v[x].size();i++){

			int ne=v[x][i];

			if(dp[x]+1>dp[ne])

				dp[ne]=dp[x]+1;

			d[ne]--;

			if(d[ne]==0)

				st.push(ne);

		}

	}

	int maxn=0;

	for(int i=1;i<=n;i++)

		maxn=max(maxn,dp[i]);

	cout<<maxn;

	return 0;

}


// Faster C++ solution for the same problem (measured speedup: 3.481631873085957x):
#include <bits/stdc++.h>



using namespace std;



#define REP(i,a,b) for(ll i = a; i < b; i++)

#define DB( x ) cout << #x << " = " << x << endl;

#define sz(s) ((int)((s).size()))

#define endl '\n'



typedef long long ll;

typedef unsigned long long ull;

typedef pair<ll, ll> pii;

typedef pair<int, pii> tri;

typedef vector <vector <ll>> matrix;

typedef pair<string, string> pss;



const ll mod = (ll)1e9 + 7;

const int MAX = (int)3e3 + 100;



const int oo = 0x3f3f3f3f;



int main() {

    ios_base::sync_with_stdio(0);

    cin.tie(0);



    int n, m; cin >> n >> m;



    vector<vector<int> > g(n + 1);



    vector<int> in(n + 1, 0);



    vector<ll> dp(n + 1, 0);



    for(int i = 0;i < m;++i){

        int a, b; cin >> a >> b;

        g[a].push_back(b);

        in[b]++;

    }



    queue<int> Q;



    for(int i = 1;i <= n;++i)

        if(!in[i])

            Q.push(i);



    vector<int> order;



    while(!Q.empty()){

        int curr = Q.front(); Q.pop();

        order.push_back(curr);

        for(auto newn : g[curr]){

            in[newn]--;

            if(!in[newn])

                Q.push(newn);

        }

    }



    ll solve = 0;



    for(auto node : order)

        for(auto newn : g[node])

            dp[newn] = max(dp[newn], dp[node] + 1);

    for(auto node : order)

        solve = max(solve, dp[node]);



    cout << solve << endl;

}