import java.io.*;
import java.lang.reflect.Array;
import java.util.*;
import java.util.Map.Entry;
import java.util.function.Function;
import java.util.stream.Collectors;

import static java.lang.System.*;

@SuppressWarnings("unused")
public class Main {
    FastScanner in = new FastScanner(System.in);
    PrintWriter out = new PrintWriter(System.out);
    final int MOD = (int)1e9+7;
    void printlnYN(boolean b){out.println((b ? "Yes" : "No"));}

    void solve() throws Exception{
        int N = in.nextInt(), M = in.nextInt();
        ArrayList<ArrayList<Integer>> G = new ArrayList<>();
        for(int i = 0; i < N; i++) G.add(new ArrayList<>());
        for(int i = 0; i < M; i++){
            int x = in.nextInt()-1, y = in.nextInt()-1;
            G.get(x).add(y);
        }

        ArrayList<Integer> sortedVertexes = topologicalSort(G);
        int[] dp = new int[N];
        int max = 0;
        for(int e : sortedVertexes){
            for(int to : G.get(e)){
                dp[to] = Math.max(dp[to], dp[e] + 1);
                max = Math.max(max, dp[to]);
            }
        }
        out.println(max);
    }

    ArrayList<Integer> topologicalSort(ArrayList<ArrayList<Integer>> G){
        //グラフの頂点数
        int n = G.size();
        //各頂点の入次数
        int[] num = new int[n];
        //入次数が0の頂点集合
        Deque<Integer> zeroVertexes = new ArrayDeque<Integer>();
        //ソート済みの頂点集合
        ArrayList<Integer> sortedVertexes = new ArrayList<Integer>();

        for(int i = 0; i < n; i++){
            for(int to : G.get(i)){
                num[to]++;
            }
        }

        for(int i = 0; i < n; i++){
            if(num[i] == 0) zeroVertexes.addFirst(i);
        }

        while(!zeroVertexes.isEmpty()){
            int v = zeroVertexes.removeFirst();
            sortedVertexes.add(v);
            for(int next : G.get(v)){
                num[next]--;
                if(num[next] == 0) zeroVertexes.addFirst(next);
            }
        }

        //(sortedList.size() != n)なら閉路が存在する(acyclicではない)
        if(sortedVertexes.size() != n) return null;
        else return sortedVertexes;
    }

    public static void main(String[] args) throws Exception {
        new Main().m();
    }

    void m() throws Exception {
        solve();
        out.flush();
    }

    static class FastScanner {
        Reader input;

        FastScanner() {this(System.in);}
        FastScanner(InputStream stream) {this.input = new BufferedReader(new InputStreamReader(stream));}

        int nextInt() {return (int) nextLong();}

        long nextLong() {
            try {
                int sign = 1;
                int b = input.read();
                while ((b < '0' || '9' < b) && b != '-' && b != '+') {
                    b = input.read();
                }
                if (b == '-') {
                    sign = -1;
                    b = input.read();
                } else if (b == '+') {
                    b = input.read();
                }
                long ret = b - '0';
                while (true) {
                    b = input.read();
                    if (b < '0' || '9' < b) return ret * sign;
                    ret *= 10;
                    ret += b - '0';
                }
            } catch (IOException e) {
                e.printStackTrace();
                return -1;
            }
        }

        double nextDouble() {
            try {
                double sign = 1;
                int b = input.read();
                while ((b < '0' || '9' < b) && b != '-' && b != '+') {
                    b = input.read();
                }
                if (b == '-') {
                    sign = -1;
                    b = input.read();
                } else if (b == '+') {
                    b = input.read();
                }
                double ret = b - '0';
                while (true) {
                    b = input.read();
                    if (b < '0' || '9' < b) break;
                    ret *= 10;
                    ret += b - '0';
                }
                if (b != '.') return sign * ret;
                double div = 1;
                b = input.read();
                while ('0' <= b && b <= '9') {
                    ret *= 10;
                    ret += b - '0';
                    div *= 10;
                    b = input.read();
                }
                return sign * ret / div;
            } catch (IOException e) {
                e.printStackTrace();
                return Double.NaN;
            }
        }

        char nextChar() {
            try {
                int b = input.read();
                while (Character.isWhitespace(b)) {
                    b = input.read();
                }
                return (char) b;
            } catch (IOException e) {
                e.printStackTrace();
                return 0;
            }
        }

        String nextStr() {
            try {
                StringBuilder sb = new StringBuilder();
                int b = input.read();
                while (Character.isWhitespace(b)) {
                    b = input.read();
                }
                while (b != -1 && !Character.isWhitespace(b)) {
                    sb.append((char) b);
                    b = input.read();
                }
                return sb.toString();
            } catch (IOException e) {
                e.printStackTrace();
                return "";
            }
        }

        public int[] nextIntArray(int n) {
            int[] res = new int[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextInt();
            }
            return res;
        }

        public int[] nextIntArrayDec(int n) {
            int[] res = new int[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextInt() - 1;
            }
            return res;
        }

        public int[] nextIntArray1Index(int n) {
            int[] res = new int[n + 1];
            for (int i = 0; i < n; i++) {
                res[i + 1] = nextInt();
            }
            return res;
        }

        public long[] nextLongArray(int n) {
            long[] res = new long[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextLong();
            }
            return res;
        }

        public long[] nextLongArrayDec(int n) {
            long[] res = new long[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextLong() - 1;
            }
            return res;
        }

        public long[] nextLongArray1Index(int n) {
            long[] res = new long[n + 1];
            for (int i = 0; i < n; i++) {
                res[i + 1] = nextLong();
            }
            return res;
        }

        public double[] nextDoubleArray(int n) {
            double[] res = new double[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextDouble();
            }
            return res;
        }
    }
}
/* end Main */


Measured memory_bytes: 51820