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;
#define chmax(a,b) if(a<b)a=b
#define pb push_back
const int N=1e5+5;

int n,m;
int dp[N];
vector<int> e[N];

int dfs(int x)
{
	int ans=0;
	if(dp[x]) return dp[x];
	for(int i=0;i<e[x].size();i++)
		chmax(ans,dfs(e[x][i])+1);
	return dp[x]=ans;
}

int main()
{
	
	cin>>n>>m;
	while(m--)
	{
		int u,v;
		cin>>u>>v;
		e[u].pb(v);
	}
	for(int i=1;i<=n;i++)
		e[0].pb(i);
	cout<<dfs(0)-1<<'\n';
	
	return 0;
}