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<iostream>
#include<queue>
#include<vector>
using namespace std;

vector<int> v[100005];
int dp[100005];

void DFS(int i){
	int maxn=0;
	for(auto a:v[i])
		if(dp[a]) maxn=max(maxn,dp[a]);
		else DFS(a),maxn=max(maxn,dp[a]);
	dp[i]=maxn+1;	
}

int main() {
	ios_base::sync_with_stdio(0);
	int n,m;
	cin>>n>>m;
	
	for(int i=0,l,r;i<m;i++)
		cin>>l>>r,v[l].push_back(r);
	
	
	for(int i=1;i<=n;i++)
		if(!dp[i])
			DFS(i);

	int maxn=0;
	for(int i=1;i<=n;i++)
		maxn=max(dp[i],maxn);
	cout<<maxn-1<<'\n';
}