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;
vector<int>g[100000],dp(100000,-1);
int rec(int i){
    if(dp[i]!=-1)return dp[i];
    for(auto x:g[i])dp[i]=max(dp[i],rec(x)+1);
    return dp[i]=max(dp[i],0);
}
int main(){
    int n,m,x,y,ans=0;
    cin>>n>>m;
    for(int i=0;i<m;i++){cin>>x>>y;g[x-1].push_back(y-1);}
    for(int i=0;i<n;i++)ans=max(ans,rec(i));
    cout<<ans<<endl;
    return 0;
}