#include <iostream>
#include<ctime>
#include<map>
#include<vector>
#include<queue>
using namespace std;
int n;
int m;
vector<int>graph[100005];
int dp[100005];
int rec(int i=0){

if(dp[i]!=-1){
    return(dp[i]);
}

int result=0;

for(int j=0; j<graph[i].size(); j++){
result=max(result, rec(graph[i][j])+1);
}
    dp[i] = result;
return(result);
}
int main()
{
cin>>n;
cin>>m;

for(int i=0; i<m; i++){
int a;
int b;
cin>>a>>b;
graph[a].push_back(b);
}

for(int i=0; i<=n; i++){
    dp[i]=-1;
}
int result1=0;
for(int i=0; i<=n; i++){
    result1=max(result1, rec(i));
}
cout<<result1;
    return 0;
}