#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;

#define REP(i,n) for(long long i = 0; i < (n); i++)
#define FOR(i, m, n) for(long long i = (m);i < (n); ++i)
#define ALL(obj) (obj).begin(),(obj).end()
#define SPEED cin.tie(0);ios::sync_with_stdio(false);

template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQR = priority_queue<T,vector<T>,greater<T>>;

constexpr long long MOD = (long long)1e9 + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433;

template <class T> vector<T> make_v(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto make_v(size_t N,T... t){return vector<decltype(make_v(t...))>(N,make_v(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}


vector<vector<int>> edge,redge,mp;
vector<int> dead,dist;

int main() {
    int N,M; cin >> N >> M;
    edge.resize(N);
    redge.resize(N);
    mp.resize(N,vector<int>(N,0));
    dead.resize(N,0);
    dist.resize(N,0);
    
    for(int i = 0; i < M; ++i) {
		int A,B; cin >> A >> B;
		A--,B--;
		edge[A].push_back(B);
		redge[B].push_back(A);
        mp[A][B]++;
	}

    for(int i = 0; i < N; ++i) {
        if(dead[i]) continue;
        for(int j = 0; j < N; ++j) dist[j] = 12345;
        dist[i] = 0;
        queue<int> q;
        q.push(i);
        while(q.size()) {
            int from = q.front();
            q.pop();
            for(auto to:edge[from]) {
                if(dead[to]) continue;
                if(dist[to]>dist[from]+1) {
                    dist[to] = dist[from] + 1;
                    q.push(to);
                }
            }
        }
        int idx = -1;
        for(int j = 0; j < N; ++j) {
            if(dead[j]) continue;
            if(dist[j]!=12345 && mp[j][i]) idx = j;
        }
        if(idx == -1) {
            dead[i] = 1;
            continue;
        }
        vector<int> live;
        live.push_back(i);
        while(idx != i) {
            live.push_back(idx);
            for(auto e:redge[idx]) {
                if(dist[e]+1==dist[idx]) {
                    idx = e;
                    break;
                }
            }
        }
        for(int j = 0; j < N; ++j) dead[j] = 1;
        for(auto e:live) dead[e] = 0;
    }
    vector<int> ans;
    for(int i = 0; i < N; ++i) if(!dead[i]) ans.push_back(i);
    corner(ans.size()<2,-1);
    cout << ans.size() << endl;
    for(auto e:ans) cout << e+1 << endl; 
    
    return 0;
}


Measured memory_bytes: 4352