#include <iostream>
#include<queue>
#include<vector>
#include<set>
#include<map>
#include<algorithm>
#include<cstring>
#include<string>
#include<cassert>
#include<cmath>
#include<climits>
#include<iomanip>
#include<bitset>

using namespace std;

#define MOD 1000000007
#define REP(i,n) for(ll (i)=0;(i)<(n);(i)++)
#define FOR(i,c) for(decltype((c).begin())i=(c).begin();i!=(c).end();++i)
#define ll long long
#define ull unsigned long long
#define all(hoge) (hoge).begin(),(hoge).end()
typedef pair<ll, ll> P;
const long long INF = 1LL << 60;
typedef vector<ll> Array;
typedef vector<Array> Matrix;


template<class T> inline bool chmin(T& a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
template<class T> inline bool chmax(T& a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}


//グラフ関連
struct Edge {//グラフ
	ll to, cap, rev;
	Edge(ll _to, ll _cap, ll _rev) {
		to = _to; cap = _cap; rev = _rev;
	}
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;
void add_edge(Graph& G, ll from, ll to, ll cap, bool revFlag, ll revCap) {//最大フロー求める Ford-fulkerson
	G[from].push_back(Edge(to, cap, (ll)G[to].size()));
	if (revFlag)G[to].push_back(Edge(from, revCap, (ll)G[from].size() - 1));
	//最大フロー・最小カットの場合逆辺は0にする
}

ll max_flow_dfs(Graph& G, ll v, ll t, ll f, vector<bool>& used)
{
	if (v == t)
		return f;
	used[v] = true;
	for (int i = 0; i < G[v].size(); ++i) {
		Edge& e = G[v][i];
		if (!used[e.to] && e.cap > 0) {
			ll d = max_flow_dfs(G, e.to, t, min(f, e.cap), used);
			if (d > 0) {
				e.cap -= d;
				G[e.to][e.rev].cap += d;
				return d;
			}
		}
	}
	return 0;
}

ll max_flow(Graph& G, ll s, ll t)
{
	ll flow = 0;
	for (;;) {
		vector<bool> used(G.size());
		REP(i, used.size())used[i] = false;
		ll f = max_flow_dfs(G, s, t, INF, used);
		if (f == 0) {
			return flow;
		}
		flow += f;
	}
}

//最短経路（負閉路検出）
void BellmanFord(Graph& G, ll s, Array& d, Array& negative) {//O(|E||V|)
	d.resize(G.size());
	negative.resize(G.size());
	REP(i, d.size())d[i] = INF;
	REP(i, d.size())negative[i] = false;
	d[s] = 0;
	REP(k, G.size() - 1) {
		REP(i, G.size()) {
			REP(j, G[i].size()) {
				if (d[i] != INF && d[G[i][j].to] > d[i] + G[i][j].cap) {
					d[G[i][j].to] = d[i] + G[i][j].cap;
				}
			}
		}
	}
	REP(k, G.size() - 1) {
		REP(i, G.size()) {
			REP(j, G[i].size()) {
				if (d[i] != INF && d[G[i][j].to] > d[i] + G[i][j].cap) {
					d[G[i][j].to] = d[i] + G[i][j].cap;
					negative[G[i][j].to] = true;
				}
				if (negative[i] == true)negative[G[i][j].to] = true;
			}
		}
	}
}

//最短経路(負の辺が含まれているとだめ)
void Dijkstra(Graph& G, ll s, Array& d) {//O(|E|log|V|)
	d.resize(G.size());
	REP(i, d.size())d[i] = INF;
	d[s] = 0;
	priority_queue<P, vector<P>, greater<P>> q;
	q.push(make_pair(0, s));
	while (!q.empty()) {
		P a = q.top();
		q.pop();
		if (d[a.second] < a.first)continue;
		REP(i, G[a.second].size()) {
			Edge e = G[a.second][i];
			if (d[e.to] > d[a.second] + e.cap) {
				d[e.to] = d[a.second] + e.cap;
				q.push(make_pair(d[e.to], e.to));
			}
		}
	}
}

//最短経路問題(01bfs:0,1の辺のみ，O(|E|))
void bfs01(Graph& G, ll s, Array& d) {
	d.resize(G.size());
	REP(i, d.size())d[i] = INF;
	d[s] = 0;
	deque<ll> deq;
	deq.push_front(s);
	while (deq.size() > 0) {
		ll v = deq.front();
		//cout << v << endl;
		deq.pop_front();
		for (auto e : G[v]) {
			if (d[v] + e.cap < d[e.to]) {
				d[e.to] = d[v] + e.cap;
				if (e.cap == 0) {
					deq.push_front(e.to);
				}
				else {
					deq.push_back(e.to);
				}
			}
		}
	}

}

//最短経路(負の辺を含んでいてもOK)
void WarshallFloyd(Graph& G, Matrix& d) {//O(V^3)
	d.resize(G.size());
	REP(i, d.size())d[i].resize(G.size());
	REP(i, d.size()) {
		REP(j, d[i].size()) {
			d[i][j] = INF;
		}
	}
	REP(i, G.size()) {
		REP(j, G[i].size()) {
			d[i][G[i][j].to] = G[i][j].cap;
		}
	}
	REP(i, G.size()) {
		REP(j, G.size()) {
			REP(k, G.size()) {
				chmin(d[j][k], d[j][i] + d[i][k]);
			}
		}
	}
}

bool tsort(Graph& graph, Array& order) {
	//トポロジカルソートO(E+V)
	//閉路検出にも使える
	int n = graph.size(), k = 0;
	Array in(n);
	for (auto& es : graph)
		for (auto& e : es)in[e.to]++;
	priority_queue<ll, Array, greater<ll>> que;
	REP(i, n)
		if (in[i] == 0)que.push(i);
	while (que.size()) {
		int v = que.top();
		que.pop();
		order.push_back(v);
		for (auto& e : graph[v])
			if (--in[e.to] == 0)que.push(e.to);
	}
	if (order.size() != n)return false;
	else return true;
}


class lca {
public:
	const int n = 0;
	const int log2_n = 0;
	std::vector<std::vector<int>> parent;
	std::vector<int> depth;

	lca() {}

	lca(const Graph& g, int root)
		: n(g.size()), log2_n(log2(n) + 1), parent(log2_n, std::vector<int>(n)), depth(n) {
		dfs(g, root, -1, 0);
		for (int k = 0; k + 1 < log2_n; k++) {
			for (int v = 0; v < (int)g.size(); v++) {
				if (parent[k][v] < 0)
					parent[k + 1][v] = -1;
				else
					parent[k + 1][v] = parent[k][parent[k][v]];
			}
		}
	}

	void dfs(const Graph& g, int v, int p, int d) {
		parent[0][v] = p;
		depth[v] = d;
		for (auto& e : g[v]) {
			if (e.to != p) dfs(g, e.to, v, d + 1);
		}
	}

	int get(int u, int v) {
		if (depth[u] > depth[v]) std::swap(u, v);
		for (int k = 0; k < log2_n; k++) {
			if ((depth[v] - depth[u]) >> k & 1) {
				v = parent[k][v];
			}
		}
		if (u == v) return u;
		for (int k = log2_n - 1; k >= 0; k--) {
			if (parent[k][u] != parent[k][v]) {
				u = parent[k][u];
				v = parent[k][v];
			}
		}
		return parent[0][u];
	}
};

class UnionFind {
	vector<int> data;
	int num;
public:
	UnionFind(int size) : data(size, -1), num(size) { }
	bool unionSet(int x, int y) {
		x = root(x); y = root(y);
		if (x != y) {
			if (data[y] < data[x]) swap(x, y);
			data[x] += data[y]; data[y] = x;
			num--;
		}
		return x != y;
	}
	bool findSet(int x, int y) {
		return root(x) == root(y);
	}
	int root(int x) {
		return data[x] < 0 ? x : data[x] = root(data[x]);
	}
	int size(int x) {
		return -data[root(x)];
	}
	int numSet() {
		return num;
	}
};

template<class Abel> struct PotentialUnionFind {
	vector<int> par;
	vector<int> rank;
	vector<Abel> diff_weight;

	PotentialUnionFind(int n = 1, Abel SUM_UNITY = 0) {
		init(n, SUM_UNITY);
	}

	void init(int n = 1, Abel SUM_UNITY = 0) {
		par.resize(n); rank.resize(n); diff_weight.resize(n);
		for (int i = 0; i < n; ++i) par[i] = i, rank[i] = 0, diff_weight[i] = SUM_UNITY;
	}

	int root(int x) {
		if (par[x] == x) {
			return x;
		}
		else {
			int r = root(par[x]);
			diff_weight[x] += diff_weight[par[x]];
			return par[x] = r;
		}
	}

	Abel weight(int x) {
		root(x);
		return diff_weight[x];
	}

	bool issame(int x, int y) {
		return root(x) == root(y);
	}

	bool merge(int x, int y, Abel w) {
		w += weight(x); w -= weight(y);
		x = root(x); y = root(y);
		if (x == y) return false;
		if (rank[x] < rank[y]) swap(x, y), w = -w;
		if (rank[x] == rank[y]) ++rank[x];
		par[y] = x;
		diff_weight[y] = w;
		return true;
	}

	Abel diff(int x, int y) {
		return weight(y) - weight(x);
	}
};

class SumSegTree {
private:

	int _sum(int a, int b, int k, int l, int r) {
		if (r <= a || b <= l)return 0;    // 交差しない
		if (a <= l && r <= b)return dat[k];   // a,l,r,bの順で完全に含まれる
		else {
			int s1 = _sum(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
			int s2 = _sum(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
			return s1 + s2;
		}
	}
public:
	int n, height;
	vector<int> dat;

	// 初期化（_nは最大要素数）
	SumSegTree(int _n) {
		n = 1;
		height = 1;
		while (n < _n) {
			n *= 2;
			height++;
		}
		dat = vector<int>(2 * n - 1, 0);
	}

	// 場所i(0-indexed)にxを足す
	void add(int i, int x) {
		i += n - 1; // i番目の葉ノードへ
		dat[i] += x;
		while (i > 0) { // 下から上がっていく
			i = (i - 1) / 2;
			dat[i] += x;
		}
	}

	// 区間[a,b)の総和。ノードk=[l,r)に着目している。
	int sum(int a, int b) {
		return _sum(a, b, 0, 0, n);
	}
};
class RmqTree {
private:
	ll _find(ll a, ll b, ll k, ll l, ll r) {
		//区間[l,r)の最小値を持つk番目のノードを探索
		if (r <= a || b <= l)return INF;    // 交差しない
		if (a <= l && r <= b)return dat[k];   // 完全に含む
		else {
			ll s1 = _find(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
			ll s2 = _find(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
			return min(s1, s2);
		}
	}
public:
	ll n, height;
	vector<ll> dat;

	// 初期化（_nは最大要素数）
	RmqTree(ll _n) {
		n = 1;
		height = 1;
		while (n < _n) {
			n *= 2;
			height++;
		}
		dat = vector<ll>(2 * n - 1, INF);
	}

	// i番目の値(0-indexed)をxに変更
	void update(ll i, ll x) {
		i += n - 1; // i番目の葉ノードへ
		dat[i] = x;
		while (i > 0) { // 登りながら更新
			i = (i - 1) / 2;//親ノードのインデックス
			dat[i] = min(dat[i * 2 + 1], dat[i * 2 + 2]);
			//子ノードの小さい方の値を代入
		}
	}

	// 区間[a,b)の最小値の取得
	ll find(ll a, ll b) {
		return _find(a, b, 0, 0, n);
	}
};


//nCr,nPr,modの逆元
class Combination {
public:
	Array fact;//n！
	Array inv;//n!の逆元
	ll mod;
	Combination(ll n, ll _mod) {
		mod = _mod;
		fact.resize(n + 1);
		fact[0] = 1;
		REP(i, n) {
			fact[i + 1] = (fact[i] * (i + 1LL)) % mod;
		}
		inv.resize(n + 1);
		REP(i, n + 1) {
			inv[i] = mod_inv(fact[i]);
		}
	}
	//modの逆元
	ll mod_inv(ll x) {
		ll n = mod - 2;
		ll res = 1LL;
		while (n > 0) {
			if (n & 1) res = res * x % mod;
			x = x * x % mod;
			n >>= 1;
		}
		return res;
	}
	ll nCr(ll n, ll r) {
		return ((fact[n] * inv[r] % mod) * inv[n - r]) % mod;
	}
	ll nPr(ll n, ll r) {
		return (fact[n] * inv[n - r]) % mod;
	}
	ll nHr(ll n, ll r) {
		//重複組み合わせ：n種類のものからr個取り出す組み合わせ
		return nCr(r + n - 1, r);
	}
};

//約数
void divisor(ll n, vector<ll>& ret) {
	for (ll i = 1; i * i <= n; i++) {
		if (n % i == 0) {
			ret.push_back(i);
			if (i * i != n) ret.push_back(n / i);
		}
	}
	sort(ret.begin(), ret.end());
}

//素因数分解
void factoring(ll n, map<ll, ll>& mp) {
	for (ll i = 2; i*i <= n; i++) {
		while (n % i == 0) {
			mp[i]++;
			n /= i;
		}
	}
	if (n != 1) mp[n]++;
}

//最大公約数
ll gcd(ll m, ll n) {
	if (n == 0)return m;
	return gcd(n, m % n);
}//gcd

//最小公倍数
ll lcm(ll m, ll n) {
	return m / gcd(m, n) * n;
}

//繰り返し自乗法
ll pow2(ll x, ll n) { //x^n 計算量O(logn)
	ll ans = 1ll;
	while (n > 0) {
		if ((n & 1) == 1) {
			ans = ans * x;
		}
		x = x * x; //一周する度にx, x^2, x^4, x^8となる
		n >>= 1; //桁をずらす n = n >> 1
	}
	return ans;
}
ll pow2_m(ll x, ll n, ll mod) { //x^n 計算量O(logn)
	ll ans = 1ll;
	while (n > 0) {
		if ((n & 1) == 1) {
			ans = ans * x % mod;
		}
		x = x * x % mod; //一周する度にx, x^2, x^4, x^8となる
		n >>= 1; //桁をずらす n = n >> 1
	}
	return ans;
}

vector<ll> lis_fast(const vector<ll>& a) {//最長部分増加列
	const ll n = a.size();
	vector<ll> A(n, INT_MAX);
	vector<ll> id(n);
	for (int i = 0; i < n; ++i) {
		//id[i] = distance(A.begin(), upper_bound(A.begin(), A.end(), a[i]));//最長部分非減少列
		id[i] = distance(A.begin(), lower_bound(A.begin(), A.end(), a[i]));
		A[id[i]] = a[i];
	}
	ll m = *max_element(id.begin(), id.end());
	vector<ll> b(m + 1);
	for (int i = n - 1; i >= 0; --i)
		if (id[i] == m) b[m--] = a[i];
	return b;
}

//memo
//output :	fixed  << setprecision(n) <<小数点以下n桁出力
//			setfill('0') << setw(12) <<num 0埋めで12文字出力
//func
//lower_bound(all(a),x):vector aでx以上の要素のイテレータを返す
//upper_bound(all(a,x)):vector aでxより大きい要素のイテレータを返す
//						setやmapの場合は専用関数あり
//調和関数はlogn! n/1+n/2+n/3+...

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	ll n, m;
	cin >> n >> m;

	Graph g(n);
	vector<bool> used(n, false);

	REP(i, m) {
		ll a, b;
		cin >> a >> b;
		a--; b--;
		add_edge(g, a, b, 1, false, 0);
	}
	
	REP(i, n) {
		if (!used[i]) {
			queue<pair<ll, Array>> que;
			Array tmp;
			que.push(make_pair(i, tmp));
			while (que.size() > 0) {
				ll v = que.front().first;
				Array order = que.front().second;
				que.pop();
				order.push_back(v);
				for (auto e : g[v]) {
					if (!used[e.to]) {
						//cout << v << " " << e.to<<"a"  << endl;
						used[e.to] = true;
						que.push(make_pair(e.to, order));
					}
					else {
						bool flag = false;
						//cout << v <<" "<< e.to << endl;
						REP(i, order.size()) {
							if (e.to == order[i]) {
								flag = true;
								cout << order.size() - i << endl;
							}
							if (flag) {
								cout << order[i]+1 << endl;
							}
						}
						if (flag) return 0;
					}
				}
			}
		}
	}



	cout << -1 << endl;

	return 0;
}


Measured memory_bytes: 384