#define _USE_MATH_DEFINES
#include<iostream>
#include<string>
#include<queue>
#include<cmath>
#include<map>
#include<set>
#include<list>
#include<iomanip>
#include<vector>
#include<random>
#include<functional>
#include<algorithm>
#include<stack>
#include<cstdio>
#include<cstring>
#include<bitset>
#include<unordered_map>
#include<climits>
#include<fstream>
#include<time.h>
#include<cassert>

using namespace std;
typedef long long ll;
typedef long double ld;
#define all(a) (a).begin(),(a).end()
#define EPS (1e-13)
#define bit(n,k) ((n>>k)&1)
const ll Mod = 1e9 + 7;
const ll mod = 998244353;
struct H {
	ll x, y;
	bool operator<(const H& b) const {
		if (x != b.x) return x < b.x;
		return y < b.y;
	}
	bool operator>(const H& b) const {
		if (x != b.x) return x > b.x;
		return y > b.y;
	}
	bool operator==(const H& b) const { return x == b.x && y == b.y; }
	bool operator!=(const H& b) const { return (*this) != b; }
};
struct P {
	ll pos, cost;
	bool operator<(const P& b) const { return cost < b.cost; }
	bool operator>(const P& b) const { return cost > b.cost; }
};
struct B {
	ll to, cost;
};
struct E {
	ll from, to, cost;
	bool operator<(const E& b) const { return cost < b.cost; }
	bool operator>(const E& b) const { return cost > b.cost; }
};
template<typename T, typename U>
void chmin(T& a, U b) {
	if (a > b) a = b;
}
template<typename T, typename U>
void chmax(T& a, U b) {
	if (a < b) a = b;
}
template<typename T>
T max_0(T a) {
	if (a < 0) return 0;
	return a;
}
template<typename T>
T min_0(T a) {
	if (a > 0) return 0;
	return a;
}
ll read() {
	ll u;
	ll k = scanf("%lld", &u);
	return u;
}
ll gcd(ll i, ll j) {
	if (i > j) swap(i, j);
	if (i == 0) return j;
	return gcd(j % i, i);
}
ll mod_pow(ll x, ll n, ll p) {
	ll res = 1;
	while (n > 0) {
		if (n & 1) res = res * x % p;
		x = x * x % p;
		n >>= 1;
	}
	return res;
}//x^n%p
ll bitcount(ll x) {
	int sum = 0;
	for (int i = 0; i < 20; i++)
		if ((1ll << i) & x) sum++;
	return sum;
}
/*ll fact_[200000];
ll comb(ll x, ll y, ll mod = Mod) {
	if (!fact_[0]) {
		fact_[0] = 1;
		for (int i = 1; i < 200000; i++) {
			fact_[i] = fact_[i - 1] * i % mod;
		}
	}
	if (x < y) return 0;
	return mod_pow(fact_[x], mod, mod) *
		mod_pow(fact_[x - y], mod - 2, mod) % mod *
		mod_pow(fact_[y], mod - 2, mod) % mod;
}//xCy*/
const ll Inf = 3023372036854775807;
const int inf = 1500000000;
#define int long long
//----------------------------------------------------
int n, m;
vector<int>e[2000];
signed main() {
	cin >> n >> m;
	for (int i = 0; i < m; i++) {
		int u, v;
		cin >> u >> v;
		e[u].push_back(v);
	}
	int shortest = n + 1;
	vector<int>res;
	for (int s = 1; s <= n; s++) {
		vector<int>dist(n + 1, -1);
		vector<int>prev(n + 1, -1);
		queue<int>que;
		que.push(s); dist[s] = 0;
		while (!que.empty()) {
			int v = que.front(); que.pop();
			for (auto t : e[v]) {
				if (dist[t] == -1) {
					dist[t] = dist[v] + 1;
					prev[t] = v;
					que.push(t);
				}
			}
		}
		for (int t = 1; t <= n; t++) {
			if (t == s || dist[t] == -1) continue;
			for (auto nt : e[t]) {
				if (nt == s) {
					vector<int>tmp({ s });
					int cur = t;
					while (cur != s) {
						tmp.push_back(cur);
						cur = prev[cur];
					}
					if (shortest > tmp.size()) {
						shortest = tmp.size();
						res = tmp;
					}
				}
			}
		}
	}
	if (shortest == n + 1) {
		cout << -1 << endl;
	}
	else {
		cout << shortest << endl;
		for (int i = 0; i < res.size(); i++) {
			cout << res[i] << endl;
		}
	}
}

Measured memory_bytes: 384