import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.lang.reflect.Array;
import java.util.*;

class Main{

	static void dfs(int node, int[] dist, boolean[] relaxed, ArrayList<Integer>[] neighbors, int[] indegree){
		relaxed[node] = true;
		for(int x: neighbors[node]){
			dist[x] = max(dist[x], dist[node] + 1);
			indegree[x]--;
			if(indegree[x] == 0){
				dfs(x, dist, relaxed, neighbors, indegree);
			}
		}
	}

	public static void main(String[] args) {
		FastReader f = new FastReader();
		int n = ii(f), m = ii(f);
		ArrayList<Integer>[] arr = new ArrayList[n + 1];
		for(int i=1; i<=n; i++)
			arr[i] = new ArrayList<Integer>();
		boolean[] relaxed = new boolean[n + 1];
		int[] indegree = new int[n + 1];
		int[] dist = new int[n + 1];
		int a, b;
		for(int i=0; i<m; ++i){
			a = ii(f);
			b = ii(f);
			arr[a].add(b); //directed edge (a -> b)
			indegree[b]++;
		}
		for(int i=1; i<=n; ++i){
			if(!relaxed[i] && indegree[i] == 0){
				dfs(i, dist, relaxed, arr, indegree);
			}
		}
		int M = 0;
		for(int i=1; i<=n; ++i){
			M = max(M, dist[i]);
		}
		printn(M);
	}

	static class FastReader {
		BufferedReader br;
		StringTokenizer st;

		public FastReader() {
			br = new BufferedReader(new
					InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

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

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

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

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	}

	public static int ii(FastReader fr) {
		return fr.nextInt();
	}

	static long il(FastReader fr) {
		return fr.nextLong();
	}

	static double id(FastReader fr) {
		return fr.nextDouble();
	}

	static void printArray(Object[] arr) {
		for (Object x : arr)
			print(x, " ");
		printn("");
	}

	static void print(Object s, String sep) {
		System.out.print(s + sep);
	}

	static void printn(Object s) {
		System.out.println(s);
	}

	static long[] inputArrL(int n, FastReader fr) {
		long[] arr = new long[n];
		for (int i = 0; i < n; i++)
			arr[i] = fr.nextLong();
		return arr;
	}

	static int[] inputArr(int n, FastReader fr) {
		int[] arr = new int[n];
		for (int i = 0; i < n; i++)
			arr[i] = fr.nextInt();
		return arr;
	}

	static int max(int... args) {
		int max = Integer.MIN_VALUE;
		for (int x : args) {
			max = Math.max(x, max);
		}
		return max;
	}

	static int min(int... args) {
		int min = Integer.MAX_VALUE;
		for (int x : args) {
			min = Math.min(x, min);
		}
		return min;
	}

	static long max(long... args) {
		long max = Integer.MIN_VALUE;
		for (long x : args) {
			max = Math.max(x, max);
		}
		return max;
	}

	static long min(long... args) {
		long min = Integer.MAX_VALUE;
		for (long x : args) {
			min = Math.min(x, min);
		}
		return min;
	}

	static int lenDigit(int d) {
		if (d == 0)
			return 1;
		int l = 0;
		while (d != 0) {
			l++;
			d /= 10;
		}
		return l;
	}

}



Measured memory_bytes: 72808