import java.io.*;
import java.util.*;
import java.util.Map.Entry;

public class Main {
	
	static int N;
	static int M;
	static ArrayList<ArrayList<Integer>> graph;
	static int[] dp;
	static int INF = Integer.MAX_VALUE / 2;
		
	public static void main(String[] args) throws IOException {
		FastScanner sc = new FastScanner(System.in);
		
		N = sc.nextInt();
		M = sc.nextInt();
		graph = new ArrayList<ArrayList<Integer>>();
		for(int i = 0; i < N; i++) graph.add(new ArrayList<Integer>());
		
		for(int i = 0; i < M; i++){
			int from = sc.nextInt()-1;
			int to = sc.nextInt()-1;
			graph.get(to).add(from);
		}
		
		dp = new int[N];
		int max = 0;
		Arrays.fill(dp, -1);
		
		for(int i = 0; i < N; i++){
			if(dp[i] == -1)
				solve(i);
			max = Math.max(max, dp[i]);
		}
		System.out.println(max);
		
	}//end main
	
	public static void solve(int k){
		int res = 0;
		for(int e : graph.get(k)){
			if(dp[e] == -1)
				solve(e);
			res = Math.max(res, dp[e] + 1);
		}
		dp[k] = res;
	}
	
}//end Main

class FastScanner {
    private InputStream in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    public FastScanner(InputStream in) {
		this.in = in;
	}
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() {
    	if (hasNextByte()) return buffer[ptr++];
    	else return -1;
    }
    private static boolean isPrintableChar(int c){
    	return 33 <= c && c <= 126;
    }
    public boolean hasNext() {
    	while(hasNextByte() && !isPrintableChar(buffer[ptr]))
    		ptr++; return hasNextByte();
    }
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() {
    	return Double.parseDouble(next());
    }
}


Measured memory_bytes: 54300