import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastReader in = new FastReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        GLongestPath solver = new GLongestPath();
        solver.solve(1, in, out);
        out.close();
    }

    static class GLongestPath {
        public void solve(int testNumber, FastReader s, PrintWriter out) {
            int n = s.nextInt();
            int m = s.nextInt();

            ArrayList[] list = new ArrayList[n];
            for (int i = 0; i < m; i++) {
                int from = s.nextInt() - 1;
                int to = s.nextInt() - 1;
                if (list[from] == null) {
                    list[from] = new ArrayList<Integer>();
                }

                list[from].add(to);
            }

//        dfs(n, list);
//        out.println(dfs1(0, list, -1));
            int[] in = dfs(n, list);
            int max = Integer.MIN_VALUE;
//        out.println(arrays.printArr(in));
            for (int i = 0; i < n; i++) {
                max = Math.max(max, in[i]);
            }
            out.println(max);
        }

        private int[] dfs(int n, ArrayList[] list) {
            int[] in = new int[n];
            Arrays.fill(in, -1);
            for (int i = 0; i < n; i++) {
                if (in[i] == -1) {
                    dfs1(i, list, -1, in);
                }
            }
            return in;
        }

        private int dfs1(int curr, ArrayList[] list, int par, int[] in) {
            ArrayList<Integer> list1 = list[curr];
            if (list1 == null) {
                return in[curr] = 0;
            }

            if (in[curr] != -1) {
                return in[curr];
            }
            int ans = 0;
            for (int i = 0; i < list1.size(); i++) {
                int next = list1.get(i);
                ans = Math.max(1 + dfs1(next, list, curr, in), ans);
            }

            return in[curr] = ans;
        }

    }

    static class FastReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private FastReader.SpaceCharFilter filter;

        public FastReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }
}



Measured memory_bytes: 36052