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
/*
If you want to aim high, aim high
Don't let that studying and grades consume you
Just live life young
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If I'm the sun, you're the moon
Because when I go up, you go down
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I'm working for the day I will surpass you
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*/
import java.util.*;
import java.io.*;
import java.math.*;

   public class Main
   {
      public static void main(String omkar[]) throws Exception
      {
         BufferedReader infile = new BufferedReader(new InputStreamReader(System.in));  
         StringTokenizer st = new StringTokenizer(infile.readLine());
         int N = Integer.parseInt(st.nextToken());
         int M = Integer.parseInt(st.nextToken());
         LinkedList<Integer>[] edges = new LinkedList[N+1];
         for(int i=1; i <= N; i++)
            edges[i] = new LinkedList<Integer>();
         for(int i=0; i < M; i++)
         {
            st = new StringTokenizer(infile.readLine());
            int a = Integer.parseInt(st.nextToken());
            int b = Integer.parseInt(st.nextToken());
            edges[a].add(b);
         }
         int[] dp = new int[N+1];
         Arrays.fill(dp, -1);
         for(int i=1; i <= N; i++)
            if(dp[i] == -1)
               dfs(i, dp, edges);
         int res = 0;
         for(int x: dp)
            res = Math.max(res, x);
         System.out.println(res);
      }
      public static int dfs(int curr, int[] dp, LinkedList<Integer>[] edges)
      {
         if(dp[curr] >= 0)
            return dp[curr];
         int res = 0;
         for(int next: edges[curr])
            res = Math.max(res, 1+dfs(next, dp, edges));
         return dp[curr] = res;
      }
   }