import java.util.Scanner;

public class p4017 {
	static int[][] ar;
	static int n,m,tot;
	public static void main(String[] args) {
		Scanner inScanner=new Scanner(System.in);
		n=inScanner.nextInt();
		m=inScanner.nextInt();
		ar=new int[m][2];
		for (int i = 0; i < m; i++) {
			ar[i][0]=inScanner.nextInt();
			ar[i][1]=inScanner.nextInt();
		}
		dfs(n, 1);
		System.out.println(tot);
	}
	private static void dfs(int n,int num) {
		
		tot=Math.max(tot, num);
		for (int i = 0; i < m; i++) {
			if (ar[i][1]==n) {
				dfs(ar[i][0], num+1);
			}
		}
	}
}