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;
typedef long long ll;
const int N=1e5+3;
vector<int> adj[N+1];
vector<int> dp(N+1,-1);
int go(int i){
 if(dp[i]!=-1) return dp[i];
 dp[i]=0;
 for(int v:adj[i]){
  dp[i]=max(dp[i],go(v)+1);
 }
 return dp[i];
}
int main(){
 int n,m;
 cin>>n>>m;
 for(int i=1;i<=n;i++){
  adj[0].push_back(i);
 }
 for(int i=0;i<m;i++){
  int u,v;
  cin>>u>>v;
  adj[u].push_back(v);
 }
 cout<<go(0)-1<<endl;
 return 0;
}