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;
typedef long long ll;
vector<ll> gr[100005];
ll ans=0;
ll dfs(ll curr,vector<ll> &dp)
{
	ll d=0;
	if(dp[curr]!=-1)
		return dp[curr];
	for(auto it:gr[curr])
	{
		d=max(d,dfs(it,dp));
	}
	return dp[curr]=1+d;
}
int main()
{
	ll n,m,x,y;
	cin>>n>>m;
	vector<ll> dp(n+1,-1);
	for(ll i=0;i<m;i++)
	{
		cin>>x>>y;
		gr[x].push_back(y);
	}
	for(ll i=1;i<=n;i++)
	{
		ans=max(ans,dfs(i,dp));
	}
	cout<<ans-1;
}