#include<iostream>
#include<cstdio>
#include<vector>
#include<cmath>

using namespace std;

const int M = 5e5 + 10;
vector<int> G[M];
int n , m;
int ans = 0;
int mm = 0;
void dfs(int x){
		ans = (ans+1) % 80112002; 
		for(int i = 0 ; i < G[x].size() ;i++){
			dfs(G[x][i]);
			mm = max(mm,ans);
			--ans;
		}
}

int main(){
	cin >> n >> m;
	for(int  i = 0 ; i < m ; i++){
		int u , v;
		cin >> u >> v;
		G[u].push_back(v);
	}
	dfs(1);
	cout << mm ;
	return 0;
}