import java.util.*;
import  java.io.*;
public class Main{

  int n ; 
  ArrayList<Integer> al[];

  Main(int n){
    this.n = n ;
    al = new ArrayList[n];
    for(int i=0;i<n;i++){
      al[i] = new ArrayList<Integer>();
    }
  }

  public static void dfs(int n, int i, Main obj, int []ans , boolean vis[]){
    vis[i] = true;
    int max = 0 ;

    Iterator it = obj.al[i].iterator();
    int temp = 0 ; 
    while(it.hasNext()){
      temp = (int) it.next();
      if(!vis[temp]){
        dfs(n,temp,obj,ans,vis);
      }
      max = Math.max(max , ans[temp] + 1);
    }
    ans[i] = max ;
  }

  public static int solute(int n, int m, Main obj, int []ans, boolean []vis){
    for(int i=0;i<n;i++){
      if(!vis[i]){
        dfs(n,i,obj,ans,vis);
      }
    }
    int max=0;
    for(int i=0;i<n;i++){
      max = Math.max(max, ans[i]);
    }
    return max ;
  }

  
  public static void main(String []args) throws Exception{
    Reader obj = new Reader();
    int n = obj.nextInt();
    int m = obj.nextInt();
    Main o = new Main(n);

    int a , b;
    for(int i = 0 ; i < m ; i++){
      a = obj.nextInt()-1;
      b = obj.nextInt()-1;
      o.al[a].add(b);
    }
    int []ans = new int[n];
    boolean []vis = new boolean[n];
    System.out.println(solute(n,m,o,ans,vis));
  }


  static class Reader
    {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;

        public Reader()
        {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public Reader(String file_name) throws IOException
        {
            din = new DataInputStream(new FileInputStream(file_name));
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public String readLine() throws IOException
        {
            byte[] buf = new byte[64]; // line length
            int cnt = 0, c;
            while ((c = read()) != -1)
            {
                if (c == '\n')
                    break;
                buf[cnt++] = (byte) c;
            }
            return new String(buf, 0, cnt);
        }

        public int nextInt() throws IOException
        {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do
            {
                ret = ret * 10 + c - '0';
            }  while ((c = read()) >= '0' && c <= '9');

            if (neg)
                return -ret;
            return ret;
        }

        public long nextLong() throws IOException
        {
            long ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');
            if (neg)
                return -ret;
            return ret;
        }

        public double nextDouble() throws IOException
        {
            double ret = 0, div = 1;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();

            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');

            if (c == '.')
            {
                while ((c = read()) >= '0' && c <= '9')
                {
                    ret += (c - '0') / (div *= 10);
                }
            }

            if (neg)
                return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException
        {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() throws IOException
        {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }

        public void close() throws IOException
        {
            if (din == null)
                return;
            din.close();
        }
    }


}


Measured memory_bytes: 53600