#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
#define rep(i,n) for(int i=0; i<(n); i++)

int N;
int M;
vector<int> num_edges;
vector<vector<int>> E;

int main() {
    cin >> N;
    cin >> M;
    E.resize(N);
    num_edges.assign(N, 0);
    rep(i,M){
        int u,v; cin >> u >> v; u--; v--;
        E[v].push_back(u);
        num_edges[u]++;
        num_edges[v]++;
    }
    rep(i,N) if(num_edges[i] % 2 == 1){ cout << "5\n"; return 0; }
    rep(i,N) sort(E[i].rbegin(), E[i].rend());

    vector<int> vertices = { 0 };
    vector<int> face_colors = { 0 };
    for(int p=1; p<N; p++){
        for(int e : E[p]){
            int num_vertices = 2;
            int color = 1 - face_colors.back();
            //cout << "p = " << p << ", e = " << e << ", v : [";
            //for(auto v : vertices) cout << " " << v; cout << " ], c : [";
            //for(auto c : face_colors) cout << " " << c; cout << " ]" << endl;
            while(true){
                if(vertices.back() <= e) break;
                num_vertices++;
                vertices.pop_back();
                face_colors.pop_back();
                if(color == face_colors.back()){ cout << "5\n"; return 0; }
            }
            //cout << "num_vertices = " << num_vertices << endl;
            if(num_vertices % 2 == 1 && color != 0){ cout << "5\n"; return 0; }
            //cout <<"color = " << color << endl;
            if(vertices.back() == e){
                face_colors.back() = color;
            }
            else{
                vertices.push_back(e);
                face_colors.push_back(color);
            }
        }
        vertices.push_back(p);
        face_colors.push_back(0);
    }
    {
        int num_vertices = vertices.size();
        int color = 1;
        if(num_vertices % 2 == 1){ cout << "5\n"; return 0; }
        for(auto c : face_colors) if(c == color){ cout << "5\n"; return 0; }
    }
    cout << "4\n";
    return 0;
}


struct ios_do_not_sync{
    ios_do_not_sync(){
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} ios_do_not_sync_instance;