using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;

namespace EDP_G
{
    class Program
    {
        static void Solve()
        {
            var n = Input.NextInt();
            var m = Input.NextInt();

            var nodes = Enumerable.Range(0, n)
                .Select(_ => new List<int>()).ToList();
            // 各ノードに入ってくるエッジの数
            var nodeEnter = Enumerable.Repeat(0, n).ToList();

            foreach (var _ in Enumerable.Range(0, m))
            {
                // 0-based
                var x = Input.NextInt() - 1;
                var y = Input.NextInt() - 1;

                nodes[x].Add(y);
                nodeEnter[y]++;
            }

            List<int> dp = bfs(n, nodes, nodeEnter);

            Console.WriteLine(dp.Max());
        }

        private static List<int> bfs(int n, List<List<int>> nodes, List<int> nodeEnter)
        {
            var q = new Queue<int>();
            foreach (var item in nodeEnter
                .Select((x, i) => new { V = x, I = i })
                .Where(x => x.V == 0))
            {
                q.Enqueue(item.I);
            }

            // bfs-main
            var dp = Enumerable.Repeat(0, n).ToList();
            while (q.Any())
            {
                var v = q.Dequeue();
                foreach (var node in nodes[v])
                {
                    // エッジ削除
                    nodeEnter[node]--;
                    if (nodeEnter[node] == 0)
                    {
                        q.Enqueue(node);
                        dp[node] = Math.Max(dp[node], dp[v] + 1);
                    }
                }
            }

            return dp;
        }

        #region Competitive Template
        public static void Main(string[] args)
        {
            var needsFlushOutput = true;
            if (needsFlushOutput)
            {
                // 細かく出力しないようにする
                var sw = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
                Console.SetOut(sw);
            }

            // 仮想的に標準入力をセットする
            // NextLine系を使っていると使えない
            //Input.SetText("");

            Solve();

            Console.Out.Flush();
        }

        static class Input
        {
            static char[] separator = { ' ' };
            public static bool IsEof { get; set; }
            static Queue<string> q { get; set; }

            static Input()
            {
                IsEof = false;
                q = new Queue<string>();
            }

            /// <summary>
            /// 入力予約
            /// </summary>
            /// <param name="items"></param>
            public static void SetText(IEnumerable<string> items)
            {
                foreach (var item in items)
                {
                    SetText(item);
                }
            }

            /// <summary>
            /// 入力予約
            /// </summary>
            /// <param name="s"></param>
            /// <returns></returns>
            public static bool SetText(string s)
            {
                if (s == null) return false;

                foreach (var elem in s.Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries))
                {
                    q.Enqueue(elem);
                }

                return true;
            }

            /// <summary>
            /// 内部queueに入力からの値をsplitして格納する
            /// </summary>
            /// <returns></returns>
            static bool read()
            {
                var s = Console.ReadLine();
                if (s == null) return false;

                foreach (var elem in s.Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries))
                {
                    q.Enqueue(elem);
                }

                if (!q.Any()) return read();

                return true;
            }

            /// <summary>
            /// 次のstringを一つ読み込む
            /// </summary>
            /// <returns></returns>
            public static string Next()
            {
                if (!q.Any())
                {
                    if (!read())
                    {
                        IsEof = true;
                        return "";
                    }
                }

                return q.Dequeue();
            }

            public static int NextInt() => int.Parse(Next());
            public static long NextLong() => long.Parse(Next());
            public static double NextDouble() => double.Parse(Next());

            public static List<string> Next(int n) => Enumerable.Range(0, n).Select(_ => Next()).ToList();
            public static List<int> NextInt(int n) => Next(n).Select(x => int.Parse(x)).ToList();
            public static List<long> NextLong(int n) => Next(n).Select(x => long.Parse(x)).ToList();
            public static List<double> NextDouble(int n) => Next(n).Select(x => double.Parse(x)).ToList();

            public static List<string> NextLine() => Console.ReadLine().Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries).ToList();
            public static List<int> NextIntLine() => NextLine().Select(x => int.Parse(x)).ToList();
            public static List<long> NextLongLine() => NextLine().Select(x => long.Parse(x)).ToList();
            public static List<double> NextDoubleLine() => NextLine().Select(x => double.Parse(x)).ToList();
        }
        #endregion
    }
}


Measured memory_bytes: 28408