Problem: 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

Solution (in CPP):

Solution:
#include<bits/stdc++.h>
using namespace std;
int n,m;
vector<int> pred[100010];
int f[100010];
int lp(int x){
	if(f[x]!=-1)
	   return f[x];
	for(int i=0;i<pred[x].size();i++){
		f[x]=max(f[x],lp(pred[x][i])+1);
	}
	f[x]=max(f[x],0);
	return f[x];
}
int main()
{
	cin>>n>>m;
	int a,b;
	for(int i=1;i<=m;i++){
		cin>>a>>b;
		pred[b].push_back(a);
	}
	int maxx=0;
	memset(f,-1,sizeof(f));
	for(int i=1;i<=n;i++){
		if(f[i]==-1)
	      f[i]=lp(i);
	    maxx=max(maxx,f[i]);
	}
	cout<<maxx<<endl;
	return 0;
}