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 va[100010];
vector<int>v[100010];
int val(int a)
{
    int &r=va[a];
    if(r!=-1)
        return r;
    r=0;
    for(auto x:v[a])
        r=max(r,val(x));
    r++;
    return r;
}
int main()
{
    memset(va,-1,sizeof va);
    int n,m,a,b;
    cin>>n>>m;
    for(int i=0; i<m; i++)
    {
        cin>>a>>b;
        v[b].push_back(a);
    }
    int ans=1;
    for(int i=1; i<=n; i++)
        ans=max(ans,val(i));
    cout<<ans-1<<endl;
}