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<vector>
using namespace std;

int n,m,u,v,dp[100007],ans;

vector<int>ho[100007];
int dfs(int p){
	int sum=0;
	if(dp[p]!=0){
		return dp[p];
	}
	for(int i=0;i<ho[p].size();i++){
		sum=max(sum,dfs(ho[p][i]));
	}
	return dp[p]=sum+1;
}

int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++){
		scanf("%d%d",&u,&v);
		ho[u].push_back(v);
	}
	for(int i=1;i<=n;i++){
		ans=max(ans,dfs(i));
	}
	printf("%d\n",ans-1);
}