#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> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(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;}

//重みなし有向グラフの最小閉路を求めるクラス
class MinimumDirectedClosedCircuit {
	vector<vector<int>> edge,redge;  
	vector<int> dist;
	size_t N;
public:
	MinimumDirectedClosedCircuit(size_t N) : N(N),edge(N),redge(N),dist(N) {
		//do nothing
	}
	void makeEdge(int from,int to){
		edge[from].push_back(to);
        redge[to].push_back(from);
	}
	//rootを含む最小閉路の集合を返す O(N)
	vector<int> solve(int root,int inf = 123456789){
		int mini = inf;
        int last = -1;
		for(int i = 0; i < N; ++i) dist[i] = -1;
		queue<int> q;
		q.push(root);
		dist[root] = 0;
		while (q.size()) {
			int curr = q.front();
			q.pop();
			for(int next:edge[curr]){
                if(next == root && mini > dist[curr]+1) mini = dist[curr]+1,last = curr;
				if(dist[next]==-1) {
					dist[next] = dist[curr] + 1;
					q.push(next);
				}
			}
		}
        vector<int> res;
        if(last != -1){
            res.push_back(last);
            q.push(last);
            while(q.size()){
                int curr = q.front();
                q.pop();
                for(int next:redge[curr]){
                    if(dist[next]+1==dist[curr]) {
                        res.push_back(next);
                        q.push(next);
                        break;
                    }
                }
            }
            reverse(res.begin(),res.end());
        }
		return res;
	}
};

int main() {
    int N,M; cin >> N >> M;
    MinimumDirectedClosedCircuit mdcc(N);
    for(int i = 0; i < M; ++i){
        int u,v; cin >> u >> v;
        u--,v--;
        mdcc.makeEdge(u,v);
    }
    int mini = 1234567;
    vector<int> ans;
    for(int i = 0; i < N; ++i){
        auto v = mdcc.solve(i);
        if(v.empty()) continue;
        if(mini>v.size()){
            mini = v.size();
            ans = v;
        }
    }
    corner(ans.empty(),-1);
    cout << ans.size() << endl;
    for(int e:ans) cout << e+1 << endl;

    return 0;
}


Measured memory_bytes: 384