#include <fstream>
#include <queue>
#include <vector>
using namespace std;
ifstream fin("dfs.in");
ofstream fout("dfs.out");
vector <int>v[10001];
queue <int>q;
int lg[10001];
char viz[10001];
void DFS(int i,int &dist)
{
    int n=v[i].size();
    int k;
    for(int j=n-1;j>=0;j--)
    {
        if(viz[j]==1);
        else
        {
            k=v[i][j];
            v[i].erase(v[i].begin()+j);
            viz[j]=1;dist++;
            DFS(k,dist);
        }
    }
    lg[i]=dist;
}
int main()
{
    int dist=0,n,m,i,x,y,maxim=0;
    fin>>n>>m;
    for(i=1;i<=m;i++)
    {
        fin>>x>>y;
        v[x].push_back(y);
    }
    for(i=1;i<=n;i++)
    {
        if(viz[i]==0)
        {
            dist=1;
            DFS(i,dist);
            lg[i]=dist;
        }
        maxim=max(maxim,lg[i]);
    }
    fout<<maxim;
    return 0;
}