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



/**

       return NO_PAIN ? NO_GAIN : GAIN;

*/



using namespace std;



#define all(v)   v.begin(), v.end()

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

#define Unique(x) x.erase(unique(all(x)), x.end())

#define endl "\n"

#define pb push_back

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

#define isOdd(n)    (n&1)

#define pow(n, m) (ll)powl(n, m)

#define numOfDigits(n)  ((n)? (ll)(log10(n))+1 : 1)

#define clr(x, val)    memset(x, val, sizeof(x)) //be careful of value and TLE

#define MOD 1000000007

#define EPS 1e-7

#define LSOne(x)  (x & (-x))   //least significant 1

typedef long long ll;

typedef long double ld;

typedef pair<int, int> pii;

typedef vector<int> vi;



const int N = 1e5 + 1;

vector<vi> adj(N);

bool vis[N];

int dp[N];

void dfs(int u){

    /*

    for each vertex, ask it what is the longest path for you.

     then his parent's longest path is max + 1

    */

    vis[u] = 1;

    for(int i = 0; i<sz(adj[u]); i++){

        int v = adj[u][i];

        if(!vis[v])

            dfs(v);

        dp[u] = max(dp[u], dp[v] + 1);

    }

}

int main()

{

    START

    //freopen("test cases\\in.txt", "r", stdin) ;

    //freopen("test cases\\in.txt", "w", stdout);

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



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

        int u, v; cin>>u>>v;

        adj[u].pb(v);

    }

    int mx = 0;

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

        if(!vis[i] && sz(adj[i]))

            dfs(i);



    cout<<*max_element(dp, dp+N);

	return 0;

}


// Faster C++ solution for the same problem (measured speedup: 1.115191927237189x):
#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;

}