import java.io.*;
import java.util.*;

public class LongestPath
{
    public static int memo[] = new int[100005];
    public static ArrayList<Integer>[] adj = new ArrayList[100005];

    public static int fun(int vertex)
    {
        if (adj[vertex].size() == 0)
            return 0;
        if (memo[vertex] != 0)
            return memo[vertex];
        for (Integer nextNode : adj[vertex])
            memo[vertex] = Math.max(memo[vertex], fun(nextNode) + 1);
        return memo[vertex];
    }

    public static void main(String[] args) throws IOException
    {
        for (int i = 0; i < 100005; i++)
            adj[i] = new ArrayList();
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String[] tmp = in.readLine().split(" ");
        int N = Integer.parseInt(tmp[0]);
        int M = Integer.parseInt(tmp[1]);
        for (int i = 0; i < M; i++)
        {
            tmp = in.readLine().split(" ");
            int x = Integer.parseInt(tmp[0]);
            int y = Integer.parseInt(tmp[1]);
            adj[x].add(y);
        }
        int ans = 0;
        for (int i = 1; i <= N; i++)
            ans = Math.max(ans, fun(i));
        System.out.println(ans);
    }
}