#include<bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")

const int N = 10004;
int n,m;
vector<int> adj[N];
int kh[2*N];
int main(){
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	cin>>n>>m;
	int ans=3;
	/*vector<int> v;
	for(int i=1;i<=n;i++){
		v.push_back(i);
	}
	random_shuffle(v.begin(),v.end());
	*/
	for(int i=0;i<m;i++){
		int x,y;
		cin>>x>>y;
		//x = v[x-1];
		adj[x].emplace_back(y);
		//adj[y].push_back(x);
	}
	for(int i=1;i<=n;++i){
		for(int x:adj[i]){
			kh[x] = i;
		}
		int cnt = 0;
		for(int j=i+1;j<=n;++j){
			for(int x:adj[j]){
				cnt += kh[x] == i?1:0;
			}
			if(cnt > 1){
				ans = 2;
				i = n+1;
				break;
			}
			cnt=0;
		}
	}
	cout<<ans<<endl;
	
	return 0;
}