// Slower C++ solution:
#include <bits/stdc++.h>

using namespace std;

#define N (int)2e5

#define fi first

#define se second

#define mp make_pair

#define pb push_back

int n,m,memo[N];

vector <int> adj[N];



int dp(int u) {

    if (adj[u].size() == 0) return 0;

    int &sol = memo[u];

    if (sol != -1) return sol;

    int keep = INT_MIN;

    for (int v : adj[u]) {

        keep = max(

            keep ,

            dp(v) + 1

        );

    }

    return sol = keep;

}



int main() {

    scanf("%d%d" , &n , &m);

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

        int u,v; scanf("%d%d" , &u , &v);

        adj[u].pb(v);

    }

    memset (memo , -1 , sizeof memo);

    int sol = INT_MIN;

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

        sol = max(sol , dp(u));

    printf("%d\n" , sol);

}


// Faster C++ solution for the same problem (measured speedup: 2.074574499575437x):
#include <bits/stdc++.h>

using namespace std;



typedef long double LD;

typedef long long ll;

typedef map<int,int> mii;

typedef pair<int,int> pii;

typedef vector<pii> vpii;



#define pb              push_back

#define mp              make_pair

#define F               first

#define S               second

#define mod             1000000007

#define INF             2147483647

#define endl            "\n"

#define FasterIO        ios_base::sync_with_stdio(0);cin.tie(0); cout.tie(0) ;

#define precision(a)    cout << fixed << setprecision(a)

#define bug(a,b)        cout<<a<<"\t"<<b<<"\n";

#define Case(a,b)       cout<<"Case "<<a<<": "<<b<<"\n";

#define reset(a,b)      memset(a,b,sizeof(a));

#define For(x,n)        for(int i=x; i<n; i++)



const int MAXN = 1000005;

const int SQRTN = 1003;

const int LOGN = 22;

const double PI=acos(-1);



int strtoint(string str){

    stringstream ss(str);

    int x = 0;

    ss >> x ;

    return x ;

}

string intostr(int x){

    stringstream ss;

    ss << x;

    string str = ss.str();

    return str;

}

void rem(string &s, const string &c) {

	auto pos = s.find(c);

	if (pos == string::npos) {

		cout << -1 << endl;

		exit(0);

	}

	s.erase(0, pos + 1);

}

void println (int n){

    cout<<n<<"\n";

}

void printvec(vector <int> &a){

    for (int i=0;i<a.size();i++){

        cout<<a[i]<<" ";

    }

    cout<<"\n";

}



void printgraph(vector <int> g[],int e){

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

        cout<<i<<" --> ";

        for (int j=0;j<g[i].size();j++){

            cout<<g[i][j]<<" ";

        }

        cout<<"\n";

    }

}



void printmap (map <int,int> &mymap){

    for (auto itr = mymap.begin(); itr != mymap.end(); ++itr) {

        cout << '\t' << itr->first

             << '\t' << itr->second << '\n';

    }

    cout << "\n";

}



vector <int> v[100005];

int dp [100005];

int visited [100005];

int temp=0;





int dfs (int k){

    if(visited[k]) return dp[k];



	visited[k] = 1;

	int res = 0;



	for(int i : v[k]){

		res = max(res, dfs(i) + 1 );

	}



	return dp[k] = res;

}



void solve(){

    int n,m;

    cin>>n>>m;

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

        int x,y;

        cin>>x>>y;

        v[x].pb(y);

    }

    int ans = 0;

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

        ans = max(ans,dfs(i));

    }

    cout<<ans<<endl;



    /*for (int i=0;i<n;i++){

        cout<<dp[i]<<" ";

    }*/

    //printgraph(v,n);





}





int main(){

    //FasterIO;



    solve();

    return 0;

}