#include "bits/stdc++.h"
using namespace std;
using ll = long long;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vvvll = std::vector<vvll>;
using dd = double;
using vdd = std::vector<dd>;
using vvdd = std::vector<vdd>;
using vvvdd = std::vector<vvdd>;

const ll INF = 1LL << 45; //10e5倍してもオーバーフローしない値
const ll MOD = (ll)(1e9 + 7);// 998244353L;
const dd EPS = 1e-11;

typedef std::numeric_limits< double > dbl;

//cin,cout高速化のおまじない＋桁数指定
struct Fast{
	Fast(){
		cin.tie(0);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
	}
} fast;

#define REPS(i, S, E) for (ll i = (S); i <= (E); i++)
#define REP(i, N) REPS(i, 0, N-1)
#define DEPS(i, E, S) for (ll i = (E); i >= (S); i--)
#define DEP(i, N) DEPS(i, N-1, 0)
#define EACH(e, v) for (auto&& e : v)

template<class T> inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; }return false; }
template<class T> inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; }return false; }
template<class T> inline T MaxE(vector<T> &v, ll S, ll E){ T m=v[S]; REPS(i, S, E)chmax(m, v[i]); return m; }//v[S]～v[E]の最大値
template<class T> inline T MinE(vector<T> &v, ll S, ll E){ T m=v[S]; REPS(i, S, E)chmin(m, v[i]); return m; }//v[S]～v[E]の最小値
template<class T> inline T MaxE(vector<T> &v, ll N) { return MaxE(v, 0, N - 1); } //先頭N個中の最大値
template<class T> inline T MinE(vector<T> &v, ll N) { return MinE(v, 0, N - 1); }
template<class T> inline T MaxE(vector<T> &v) { return MaxE(v, (ll)v.size()); }
template<class T> inline T MinE(vector<T> &v) { return MinE(v, (ll)v.size()); }
template<class T> inline ll MaxI(vector<T> &v, ll S, ll E){ ll m=S; REPS(i, S, E){ if (v[i]>v[m])m=i; }return m; }
template<class T> inline ll MinI(vector<T> &v, ll S, ll E){ ll m=S; REPS(i, S, E){ if (v[i]<v[m])m=i; }return m; }
template<class T> inline ll MaxI(vector<T> &v, ll N) { return MaxI(v, 0, N - 1); }
template<class T> inline ll MinI(vector<T> &v, ll N) { return MinI(v, 0, N - 1); }
template<class T> inline ll MaxI(vector<T> &v) { return MaxI(v, (ll)v.size()); }
template<class T> inline ll MinI(vector<T> &v) { return MinI(v, (ll)v.size()); }
template<class T> inline T Sum(vector<T> &v, ll S, ll E){ T s=v[S]; REPS(i, S+1, E)s+=v[i]; return s; }
template<class T> inline T Sum(vector<T> &v, ll N) { return Sum(v, 0, N - 1); }
template<class T> inline T Sum(vector<T> &v) { return Sum(v, v.size()); }
template<class T> inline T SumMOD(vector<T> &v, ll S, ll E){ T s=v[S]; REPS(i, S+1, E)s=(s+v[i])%MOD; return s; }
template<class T> inline T SumMOD(vector<T> &v, ll N) { return SumMOD(v, 0, N - 1); }
template<class T> inline T SumMOD(vector<T> &v) { return SumMOD(v, v.size()); }

inline ll AddMOD(ll a, ll b) { return (a + b) % MOD; }
inline ll SubMOD(ll a, ll b) { return (a - b + MOD) % MOD; }
inline ll MulMOD(ll a, ll b) { return (a * b) % MOD; }

inline ll in() { ll x; cin >> x; return x; }
inline double inD() { double x; cin >> x; return x; }
inline string inS() { string x; cin >> x; return std::move(x); }
template<class T=ll> inline vector<T> in_v(ll N){ vector<T> v(N); REP(i, N)cin>>v[i]; return move(v); }
inline void To0origin(vll &v) { EACH(e, v) e--; }

ll N, M;
vvll to;
vll state; //0:未調査、1:seen、2:done
vll heiro;
vector<bool> onHeiro;

bool dfs(ll v){
	state[v] = 1;//seen

	for (auto &&u: to[v]){
		if (state[u] == 1){//doneする前に再訪したらループ！
			heiro.push_back(u);
			heiro.push_back(v);
			return true;
		}
		bool ret = dfs(u);
		if (ret){//子が検出
			heiro.push_back(v);
			return true;
		}
	}
	state[v] = 2;//done

	return false;
}


int main()
{
	cin >> N >> M;
	to.resize(N);
	REP(i, M){
		ll A, B;  cin >> A >> B;
		A--; B--;
		to[A].push_back(B);
	}
	state.resize(N, 0);

	//閉路一例を得る
	bool isHeiro = false;
	REP(i, N){
		isHeiro = dfs(i);
		if (isHeiro) break;
	}
	if (!isHeiro) {
		cout << -1 << '\n';
		return 0;
	}

	ll heiroLen = (ll)heiro.size();
	REPS(i, 1, heiroLen-1){
		if (heiro[i] != heiro[0]) continue;

		heiro.resize(i);
		break;
	}
	std::reverse(heiro.begin(), heiro.end());
	heiroLen = (ll)heiro.size();

	//現閉路上の印
	onHeiro.resize(N, false);
	for (auto &&v: heiro) onHeiro[v] = true;

	while (true){
		//閉路上ノードで出自数2があるかチェック
		heiroLen = (ll)heiro.size();
		ll vv = -1;
		ll uu = -1;
		REP(i, heiroLen){
			ll v = heiro[i];
			ll nxv = heiro[(i+1)%heiroLen];
			for (auto &&u: to[v]){
				if (!onHeiro[u]) continue;
				if (u == nxv) continue;
				uu = u;
				vv = v;
				break;
			}
			if (uu != -1) break;
		}

		if (uu == -1) break;//出自数2なし→検出完了
		
		//出自数2以上あり → 短絡させ新しい閉路を作る
		vll heiroTmp;
		ll i;
		for (i=0; i<heiroLen; i++){
			if (heiro[i] == uu) break;
		}
		for (;; i++){
			ll u = heiro[i%heiroLen];
			heiroTmp.push_back(u);
			if (u == vv) break;
		}
		i++;
		for (;; i++){
			ll u = heiro[i%heiroLen];
			if (u == uu) break;
			onHeiro[u] = false;
		}
		heiro = heiroTmp;
	}

	cout << heiro.size() << '\n';
	for (auto &&e: heiro){
		cout << e+1 << '\n';
	}
	return 0;
}


Measured memory_bytes: 384