There is a directed graph G with N vertices and M edges. The vertices are numbered 1, 2, \ldots, N, and for each i (1 \leq i \leq M), the i-th directed edge goes from Vertex x_i to y_i. G does not contain directed cycles.

Find the length of the longest directed path in G. Here, the length of a directed path is the number of edges in it.

Constraints

* All values in input are integers.
* 2 \leq N \leq 10^5
* 1 \leq M \leq 10^5
* 1 \leq x_i, y_i \leq N
* All pairs (x_i, y_i) are distinct.
* G does not contain directed cycles.

Input

Input is given from Standard Input in the following format:


N M
x_1 y_1
x_2 y_2
:
x_M y_M


Output

Print the length of the longest directed path in G.

Examples

Input

4 5
1 2
1 3
3 2
2 4
3 4


Output

3


Input

6 3
2 3
4 5
5 6


Output

2


Input

5 8
5 3
2 3
2 4
5 2
5 1
1 4
4 3
1 3


Output

3


import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Scanner;

public class Main {

	private static Scanner in;
	
	private static int n;
	private static int m;
	private static HashMap<Integer, ArrayList<Integer>> g;
	private static int[] dp;
	
	public static void main(String[] args) {
		in = new Scanner(System.in);
		n = in.nextInt();
		m = in.nextInt();
		g = new HashMap<>();
		for(int i=0; i<n; i++) {
			g.put(i, new ArrayList<>());
		}
		
		dp = new int[n];
		
		Arrays.fill(dp, -1);
		
		for(int i=0; i<m; i++) {
			int p = in.nextInt()-1;
			int q = in.nextInt()-1;
			g.get(p).add(q);
//			g.get(q).add(p);
		}
		
		
		int ans = 0;
		for(int i=0; i<n; i++) ans = Math.max(ans, solve(i));
		
		System.out.println(ans);
	
	}
	
	private static int solve(int i) {
		
		if (dp[i]!=-1) return dp[i];
		
		int ans = 0;
		
		for(int v: g.get(i)) {
			ans = Math.max(ans, 1+solve(v));
		}
		
		return dp[i]=ans;
	}

}
