using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using static System.Console;
using static System.Convert;
using static System.Math;
//using Debug;
//using static System.Globalization.CultureInfo;
using System.Text;
class Program
{
    static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);
        Solve();
        Console.Out.Flush();
        //WriteLine(Solve());
    }
    static void Solve()
    {
        var nm = Input.ar;
        var adj = Input.edge<int>(nm[0]);
        while (nm[1]-- > 0)
        {
            var xy = Input.ar;
            xy[0] --;xy[1]--;
            adj[xy[0]].Add(xy[1]);
        }
        var list = TopologicalSort(adj);
        var dp = new int[nm[0] + 1];
        for (var i = 0; i < nm[0]; i++)
            foreach (var ad in adj[list[i]])
                dp[ad] = Max(dp[ad],dp[list[i]] + 1);
        WriteLine(dp.Max());
    }
    public static List<int> TopologicalSort(IList<IEnumerable<int>> adj)
    {
        var res = new List<int>(adj.Count);
        var th = new bool[adj.Count];
        for (var i = 0; i < adj.Count; i++)
            if (!th[i])
                Topodfs(adj, i, th, res);
        res.Reverse();
        return res;
    }
    private static void Topodfs(IList<IEnumerable<int>> adj, int index, bool[] th, List<int> res)
    {
        th[index] = true;
        foreach (var ad in adj[index])
            if (!th[ad])
                Topodfs(adj, ad, th, res);
        res.Add(index);
    }
}

public class Input
{
    public static string read => ReadLine().Trim();
    public static int[] ar => read.Split(' ').Select(int.Parse).ToArray();
    public static int num => ToInt32(read);
    public static long[] arL => read.Split(' ').Select(long.Parse).ToArray();
    public static long numL => ToInt64(read);
    public static char[][] gred(int h)
        => Enumerable.Repeat(0, h).Select(_ => read.ToCharArray()).ToArray();
    public static int[] ar1D(int n)
        => Enumerable.Repeat(0, n).Select(_ => num).ToArray();
    public static long[] arL1D(int n)
        => Enumerable.Repeat(0, n).Select(_ => numL).ToArray();
    public static string[] strs(int n)
        => Enumerable.Repeat(0, n).Select(_ => read).ToArray();
    public static int[][] ar2D(int n)
        => Enumerable.Repeat(0, n).Select(_ => ar).ToArray();
    public static long[][] arL2D(int n)
        => Enumerable.Repeat(0, n).Select(_ => arL).ToArray();
    public static List<T>[] edge<T>(int n)
        => Enumerable.Repeat(0, n).Select(_ => new List<T>()).ToArray();
    public static T getValue<T>(string g)
    {
        var t = typeof(T);
        if (t == typeof(int))
            return (T)(object)int.Parse(g);
        if (t == typeof(long))
            return (T)(object)long.Parse(g);
        if (t == typeof(string))
            return (T)(object)g;
        if (t == typeof(char))
            return (T)(object)char.Parse(g);
        if (t == typeof(double))
            return (T)(object)double.Parse(g);
        if (t == typeof(bool))
            return (T)(object)bool.Parse(g);
        return default(T);
    }
    public static void Make<T1, T2>(out T1 v1, out T2 v2)
    {
        v1 = Next<T1>();
        v2 = Next<T2>();
    }
    public static void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3)
    {
        Make(out v1, out v2);
        v3 = Next<T3>();
    }
    public static void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4)
    {
        Make(out v1, out v2, out v3);
        v4 = Next<T4>();
    }
    public static void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5)
    {
        Make(out v1, out v2, out v3, out v4);
        v5 = Next<T5>();
    }
    private static Queue<string> sc = new Queue<string>();
    public static T Next<T>(){ if (sc.Count == 0) foreach (var item in read.Split(' ')) sc.Enqueue(item);return getValue<T>(sc.Dequeue()); }
    public const long Inf = (long)1e18;
    public const double eps = 1e-6;
    public const string Alfa = "abcdefghijklmnopqrstuvwxyz";
    public const int MOD = 1000000007;
}


Measured memory_bytes: 33996