import java.io.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.StringTokenizer;

public class Main {

    static class Task {

        private List<Integer>[] adj;
        private int[] memo;

        private int dfs(int idx) {
            if (memo[idx] != -1) {
                return memo[idx];
            }

            List<Integer> nextList = adj[idx];
            if (nextList == null) {
                return 0;
            }

            int ansMax = 0;
            for (Integer next: nextList) {
                ansMax = Math.max(ansMax, dfs(next));
            }

            memo[idx] = ansMax + 1;
            return memo[idx];
        }

        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            adj = new ArrayList[n + 1];
            for (int i = 0; i < m; i++) {
                int v1 = in.nextInt();
                int v2 = in.nextInt();
                if (adj[v1] == null) {
                    adj[v1] = new ArrayList<>();
                }
                adj[v1].add(v2);
            }

            memo = new int[n + 1];
            Arrays.fill(memo, -1);
            int ans = 0;
            for (int i = 1; i <= n; i++) {
                ans = Math.max(ans, dfs(i));
            }

            out.println(ans);
        }
    }

    private static void sort(double[] arr) {
        Double[] objArr = Arrays.stream(arr).boxed().toArray(Double[]::new);
        Arrays.sort(objArr);
        for (int i = 0; i < arr.length; i++) {
            arr[i] = objArr[i];
        }
    }

    private static void sort(int[] arr) {
        Integer[] objArr = Arrays.stream(arr).boxed().toArray(Integer[]::new);
        Arrays.sort(objArr);
        for (int i = 0; i < arr.length; i++) {
            arr[i] = objArr[i];
        }
    }

    private static void sort(long[] arr) {
        Long[] objArr = Arrays.stream(arr).boxed().toArray(Long[]::new);
        Arrays.sort(objArr);
        for (int i = 0; i < arr.length; i++) {
            arr[i] = objArr[i];
        }
    }

    private static void solve() {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task task = new Task();
        task.solve(1, in, out);
        out.close();
    }

    public static void main(String[] args) {
        new Thread(null, () -> solve(), "1", 1 << 26).start();
    }

    static class InputReader {

        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

    }

}


Measured memory_bytes: 58592