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;

int mem[100005];
int n,m;
vector<int> adj[100005];

int go(int x)
{
	if(mem[x]!=-1) return mem[x];
	mem[x]=0;
	for(int i:adj[x])
		mem[x]=max(mem[x],1+go(i));
	return mem[x];
}

int main()
{
	cin >> n >> m;
	for(int i=0;i<m;i++)
	{
		int x,y;
		cin >> x >> y;
		adj[x].push_back(y);
	}
	int ans=0;
	memset(mem,-1,sizeof mem);
	for(int i=1;i<=n;i++) ans=max(ans,go(i));
	cout << ans << endl;
}