import java.util.*;
import java.lang.*;
import java.io.*;
class Graph{
    int n;
    ArrayList<Integer> graph [];

    Graph(int n){
        this.n=n;
        graph = new ArrayList[n+1];
        for(int i=0;i<=n;i++){
            graph[i]=new ArrayList<>();
        }

    }

    void addEdge(int u,int v){
        graph[u].add(v);
    }
    int findLongest(){
        int dp[] =new int[n+1];
        boolean visited[] =new boolean[n+1];

        for(int i=1;i<=n;i++){
            if(!visited[i]){
                dfs(i,visited,dp);
            }
        }

        int ans=0;
        for(int i=1;i<=n;i++){
            ans=Math.max(ans,dp[i]);
        }

        return ans;
    }
    void dfs(int i,boolean[] visited,int dp[] ){
        visited[i]=true;

        for(int j=0;j<graph[i].size();j++)
        {
            if(!visited[graph[i].get(j)]){
                dfs(graph[i].get(j),visited,dp);
            }

            dp[i]=Math.max(dp[i],1+dp[graph[i].get(j)]);
        }
    }

}
public class Main{
    static FastScanner sc = new FastScanner(System.in);
    static PrintWriter out = new PrintWriter(System.out);
    public static void main (String[] args) throws Exception {
        int n=sc.nextInt();
        int q=sc.nextInt();

        Graph g=new Graph(n);

        for(int i=0;i<q;i++){
            int u=sc.nextInt();
            int v=sc.nextInt();
            g.addEdge(u,v);
        }
        
        out.println(g.findLongest());
        out.close();

    }    
    
}
class FastScanner
{

    final private int BUFFER_SIZE = 1 << 17;
    private DataInputStream din;
    private byte[] buffer;
    private int bufferPointer, bytesRead;

    public FastScanner(InputStream in)
    {
        din = new DataInputStream(in);
        buffer = new byte[BUFFER_SIZE];
        bufferPointer = bytesRead = 0;
    }
    public String nextLine() throws Exception
    {
        StringBuffer sb = new StringBuffer("");
        byte c = read();
        while (c <= ' ') c = read();
        do
        {
            sb.append((char)c);
            c = read();
        }
        while(c > ' ');
        return sb.toString();
    }

    public char nextChar() throws Exception
    {
        byte c = read();
        while(c <= ' ') c = read();
        return (char)c;
    }

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

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

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

    private byte read() throws Exception
    {
        if (bufferPointer == bytesRead) fillBuffer();
        return buffer[bufferPointer++];
    }
    public double nextDouble() throws Exception
    {
        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;
    }

    
}




Measured memory_bytes: 50188