using EducationalDPContest.Questions;
using EducationalDPContest.Extensions;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace EducationalDPContest.Questions
{
    public class QuestionG : AtCoderQuestionBase
    {
        List<int>[] graph;
        int[] dpLength;

        public override IEnumerable<object> Solve(TextReader inputStream)
        {
            var nm = inputStream.ReadIntArray();
            var n = nm[0];
            var m = nm[1];
            graph = new List<int>[n];

            for (int i = 0; i < n; i++)
            {
                graph[i] = new List<int>();
            }

            dpLength = Enumerable.Repeat(int.MinValue, n).ToArray();

            for (int i = 0; i < m; i++)
            {
                var xy = inputStream.ReadIntArray();
                graph[xy[0] - 1].Add(xy[1] - 1);
            }

            var longestLength = 0;
            for (int i = 0; i < n; i++)
            {
                UpdateWhenLarge(ref longestLength, Search(i));
            }

            yield return longestLength;
        }

        int Search(int nodeIndex)
        {
            if (dpLength[nodeIndex] >= 0)
            {
                return dpLength[nodeIndex];
            }

            int longestLength = 0;
            foreach (var nextNodeIndex in graph[nodeIndex])
            {
                UpdateWhenLarge(ref longestLength, Search(nextNodeIndex) + 1);
            }
            return dpLength[nodeIndex] = longestLength;
        }
    }
}


namespace EducationalDPContest
{
    class Program
    {
        static void Main(string[] args)
        {
            IAtCoderQuestion question = new QuestionG();    // 問題に合わせて書き換え
            var answers = question.Solve(Console.In);
            foreach (var answer in answers)
            {
                Console.WriteLine(answer);
            }
        }
    }
}

#region Base Classes

namespace EducationalDPContest.Questions
{

    public interface IAtCoderQuestion
    {
        IEnumerable<object> Solve(string input);
        IEnumerable<object> Solve(TextReader inputStream);
    }

    public abstract class AtCoderQuestionBase : IAtCoderQuestion
    {
        public IEnumerable<object> Solve(string input)
        {
            var stream = new MemoryStream(Encoding.Unicode.GetBytes(input));
            var reader = new StreamReader(stream, Encoding.Unicode);

            return Solve(reader);
        }

        public abstract IEnumerable<object> Solve(TextReader inputStream);

        protected void UpdateWhenSmall<T>(ref T value, T other) where T : IComparable<T>
        {
            if (other.CompareTo(value) < 0)
            {
                value = other;
            }
        }

        protected void UpdateWhenLarge<T>(ref T value, T other) where T : IComparable<T>
        {
            if (other.CompareTo(value) > 0)
            {
                value = other;
            }
        }
    }

}

#endregion

#region Collections

namespace EducationalDPContest.Collections
{
    public static class SearchExtension
    {
        public static int GetGreaterEqualIndex<T>(this T[] span, T minValue) where T : IComparable<T>
        {
            int ng = -1;
            int ok = span.Length;

            return BoundaryBinarySearch(span, v => v.CompareTo(minValue) >= 0, ng, ok);
        }

        public static int GetSmallerEqualIndex<T>(this T[] span, T maxValue) where T : IComparable<T>
        {
            int ng = span.Length;
            int ok = -1;

            return BoundaryBinarySearch(span, v => v.CompareTo(maxValue) <= 0, ng, ok);
        }

        private static int BoundaryBinarySearch<T>(T[] span, Predicate<T> predicate, int ng, int ok)
        {
            // めぐる式二分探索
            // Span.BinarySearchだとできそうでできない（lower_boundがダメ）
            while (Math.Abs(ok - ng) > 1)
            {
                int mid = (ok + ng) / 2;

                if (predicate(span[mid]))
                {
                    ok = mid;
                }
                else
                {
                    ng = mid;
                }
            }
            return ok;
        }
    }
}

#endregion
#region Extensions

namespace EducationalDPContest.Extensions
{
    internal static class TextReaderExtensions
    {
        internal static int ReadInt(this TextReader reader) => int.Parse(ReadString(reader));
        internal static long ReadLong(this TextReader reader) => long.Parse(ReadString(reader));
        internal static double ReadDouble(this TextReader reader) => double.Parse(ReadString(reader));
        internal static string ReadString(this TextReader reader) => reader.ReadLine();

        internal static int[] ReadIntArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(int.Parse).ToArray();
        internal static long[] ReadLongArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(long.Parse).ToArray();
        internal static double[] ReadDoubleArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(double.Parse).ToArray();
        internal static string[] ReadStringArray(this TextReader reader, char separator = ' ') => reader.ReadLine().Split(separator);
    }
}

#endregion

Measured memory_bytes: 31612