#include "bits/stdc++.h"

using namespace std;
#ifdef _DEBUG
#define _GLIBCXX_DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

#define int long long
#define ll long long
#define DBG 1
#define rep(i, a, b) for (int i = (a); i < (b); i++)
#define rrep(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define loop(n) rep(loop, (0), (n))
#define all(c) begin(c), end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9) + 7;
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
using pii = pair<int, int>;
// template<class T> ostream &operator<<(ostream &os,T &t){dump(t);return os;}
template <typename T, typename S>istream &operator>>(istream &is, pair<T, S> &p) { is >> p.first >> p.second; return is; }
//template <typename T, typename S>ostream &operator<<(ostream &os, pair<T, S> &p) {os << p.first << " " << p.second;return os;}

template <typename T> void printvv(const vector<vector<T>> &v) {
	cerr << endl;
	rep(i, 0, v.size()) rep(j, 0, v[i].size()) {
		if (typeid(v[i][j]).name() == typeid(INF).name() and v[i][j] == INF) {
			cerr << "INF";
		}
		else
			cerr << v[i][j];
		cerr << (j == v[i].size() - 1 ? '\n' : ' ');
	}
	cerr << endl;
}

#ifndef _DEBUG
#define printvv(...)
#endif
void YES(bool f) { cout << (f ? "YES" : "NO") << endl; }
void Yes(bool f) { cout << (f ? "Yes" : "No") << endl; }
template <class T, class U>bool chmin(T& a, U b) { if (a > b) { a = b; return true; }return false; }
template <class T, class U>bool chmax(T& a, U b) { if (a < b) { a = b; return true; }return false; }
using Weight = int;
using Flow = int;
struct Edge {
	int s, d; Weight w; Flow c; int id;
	Edge() {};
	Edge(int s, int d, int id) : s(s), d(d), w(1), id(id) {};
};
bool operator<(const Edge &e1, const Edge &e2) { return e1.w < e2.w; }
bool operator>(const Edge &e1, const Edge &e2) { return e2 < e1; }
inline ostream &operator<<(ostream &os, const Edge &e) { return (os << '(' << e.s << ", " << e.d << ", " << e.w << ')'); }

using Edges = vector<Edge>;
using Graph = vector<Edges>;
using Array = vector<Weight>;
using Matrix = vector<Array>;

void addArc(Graph &g, int s, int d, int id) {
	g[s].emplace_back(s, d, id);
}
void addEdge(Graph &g, int a, int b, Weight w = 1) {
	addArc(g, a, b, w);
	addArc(g, b, a, w);
}

//単一始点最短経路(負閉路なし)
//Dijkstra O((|E|+|V|)log|V|)
//dist: 始点から各頂点までの最短距離
//戻り値: 最短経路木の親頂点(根は-1)
vector<int> dijkstra(const Graph &g, int s, Array &dist) {
	int n = g.size();
	assert(s < n);
	enum { WHITE, GRAY, BLACK };
	vector<int> color(n, WHITE); color[s] = GRAY;
	vector<int> prev(n, -1);
	dist.assign(n, INF); dist[s] = 0;
	using State = tuple<Weight, int, int>; //始点からの最短距離 子 親
	priority_queue<State, vector<State>, greater<State>> pq; pq.emplace(0, s, -1);
	while (pq.size()) {
		Weight d; int v, u; tie(d, v, u) = pq.top(); pq.pop();
		if (dist[v] < d)continue;
		color[v] = BLACK; prev[v] = u;
		for (auto &e : g[v]) {
			if (color[e.d] == BLACK)continue;
			if (dist[e.d] > dist[v] + e.w) {
				dist[e.d] = dist[v] + e.w;
				pq.emplace(dist[e.d], e.d, v);
				color[e.d] = GRAY;
			}
		}
	}
	return prev;
}



signed main(signed argc, char *argv[]) {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed << setprecision(12);

	int N, M; cin >> N >> M;
	vector<int>A(M), B(M); rep(i, 0, M) { cin >> A[i] >> B[i]; A[i]--, B[i]--; }
	Graph g(N);
	rep(i, 0, M) {
		addArc(g, A[i], B[i], i);
	}

	pii p(INF,INF);
	rep(i, 0, N) {
		Array dist;
		auto prev = dijkstra(g, i, dist);
		rep(j, 0, N) {
			for (auto e : g[j]) {
				if (e.d == i) {
					chmin(p, pii(dist[j] + 1, i));
				}
			}
		}
	}
	dump(p);
	if (p.second >= N) {
		cout << -1 << endl; return 0;
	}
	Array dist;
	auto prev = dijkstra(g, p.second, dist);
	set<int>ans;
	rep(i, 0, N) {
		for (auto e : g[i]) {
			if (e.d == p.second and dist[i]+1==p.first) {
				int cur = i;
				while (prev[cur] != -1) {
					ans.insert(cur);
					cur = prev[cur];
				}
				ans.insert(cur);

				cout << ans.size() << endl;
				for (auto x : ans) {
					cout << x + 1 << endl;
				}
				exit(0);
			}
		}
	}
	cout << -1 << endl;

	return 0;
}


Measured memory_bytes: 512