#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

#define MOD 1000000007
#define MOD2 998244353
#define int long long
//#define PI 3.14159265358979

#define rep(i, n) for (int i = 0; i < (int)(n); i++)

using namespace std;

template < typename T >
ostream &operator<<(ostream &os, const vector< T > &A) {
	for (int i = 0; i < A.size(); i++)
		os << A[i] << " ";
	os << endl;
	return os;
}
template <>
ostream &operator<<(ostream &os, const vector< vector< int > > &A) {
	int N = A.size();
	int M;
	if (N > 0)
		M = A[0].size();
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < M; j++)
			os << A[i][j] << " ";
		os << endl;
	}
	return os;
}

typedef pair< int, int > pii;
typedef long long ll;

struct edge {
	int from, to, d, c;
	edge(int _from = 0, int _to = 0, int _d = 0, int _c = 0) {
		from = _from;
		to = _to;
		d = _d;
		c = _c;
	}
	bool operator<(const edge &rhs) const {
		return (d == rhs.d) ? (c < rhs.c) : (d < rhs.d);
	}
};
struct aabb {
	int x1, y1, x2, y2;
	aabb(int x1, int y1, int x2, int y2) : x1(x1), y1(y1), x2(x2), y2(y2) {}
};

typedef vector< edge > edges;
typedef vector< edges > graph;
struct flow {
	int to, cap, rev, cost;
	flow(int to = 0, int cap = 0, int rev = 0, int cost = 0) : to(to), cap(cap), rev(rev), cost(cost) {}
};
typedef vector< vector< flow > > flows;

const int di[4] = {0, -1, 0, 1};
const int dj[4] = {-1, 0, 1, 0};
const int ci[5] = {0, 0, -1, 0, 1};
const int cj[5] = {0, -1, 0, 1, 0};
const ll LINF = LLONG_MAX / 2;
const int INF = INT_MAX / 2;
const double PI = acos(-1);

int pow2(int n) { return 1 << n; }
template < typename T, typename U >
bool chmin(T &x, const U &y) {
	if (x > y) {
		x = y;
		return true;
	}
	return false;
}
template < typename T, typename U >
bool chmax(T &x, const U &y) {
	if (x < y) {
		x = y;
		return true;
	}
	return false;
}

struct initializer {
	initializer() {
		cout << fixed << setprecision(11);
	}
};
initializer _____;

int N, M, K, T, Q;
vector< int > dijkstra(graph &G, int s) {
	///stat must have "<"&">"
	///init d=vector<stat>
	vector< int > d(G.size(), LINF);
	d[s] = 0;
	//init q=p-queue<pair<stat,index> >
	priority_queue< pii, vector< pii >, greater< pii > > q;
	q.push(pii(0, s));

	while (!q.empty()) {
		pii p = q.top();
		q.pop();
		if (d[p.second] < p.first)
			continue;
		for (int i = 0; i < G[p.second].size(); i++) {
			edge e = G[p.second][i];
			if (p.first + e.d < d[e.to]) {
				d[e.to] = p.first + e.d;
				q.push(pii(d[e.to], e.to));
			}
		}
	}
	return d;
}
bool dfs(graph &G, vector< int > &done, set< int > &s, int x, int t, int minc) {
	s.insert(x);
	rep(i, G[x].size()) {
		int j = G[x][i].to;
		if (j == t && s.size() + 1 == minc) {
			s.insert(j);
			return true;
		}
		if (done[j])
			continue;
		done[j] = true;
		if (dfs(G, done, s, j, t, minc)) {
			return true;
		}
	}
	s.erase(x);
	return false;
}
signed main() {
	cin >> N >> M;
	graph G(N);
	vector< vector< int > > H(N);
	rep(i, M) {
		int A, B;
		cin >> A >> B;
		--A;
		--B;
		G[A].push_back(edge(A, B, 1));
		H[B].push_back(A);
	}
	int minc = LINF, mins = -1, mint = -1;
	rep(s, N) {
		vector< int > d = dijkstra(G, s);
		rep(i, H[s].size()) {
			int t = H[s][i];
			if (d[t] != LINF) {
				if (d[t] + 1 < minc) {
					minc = d[t] + 1;
					mins = s;
					mint = t;
				}
			}
		}
	}
	if (mint == -1) {
		cout << -1 << endl;
		return 0;
	}
	set< int > s;
	vector< int > done(N, 0);
	dfs(G, done, s, mins, mint, minc);
	cout << s.size() << endl;
	for (auto it = s.begin(); it != s.end(); it++) {
		cout << (*it) + 1 << endl;
	}
	return 0;
}

Measured memory_bytes: 512