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>
#define ll long long
using namespace std;
const int mxN=1e6;
 vector<int >v[mxN];
int dp[mxN];
int vis[mxN];
int n,m;
void solve(int i){
	vis[i]=1;
	for(auto t:v[i]){
		if(!vis[t]){
			solve(t);
			}
		dp[i]=max(dp[i],dp[t]+1);
		}
	}
int main(){
	
	cin>>n>>m;
	memset(dp,0,sizeof(dp));
	memset(vis,0,sizeof(vis));
	for(int i=0;i<m;i++){
		int x,y;
		cin>>x>>y;
		v[x].push_back(y);
		}
	
	for(int i=1;i<=n;i++){
		if(!vis[i]){
		solve(i);
		}
	
	}
	cout<<*max_element(dp,dp+n+1);
	return 0;
	}