/* #region Head */

#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pll = pair<ll, ll>;
template <class T> using vc = vector<T>;
template <class T> using vvc = vc<vc<T>>;
using vll = vc<ll>;
using vvll = vvc<ll>;
using vld = vc<ld>;
using vvld = vvc<ld>;
using vs = vc<string>;
using vvs = vvc<string>;
template <class T, class U> using um = unordered_map<T, U>;
template <class T> using pq = priority_queue<T>;
template <class T> using pqa = priority_queue<T, vc<T>, greater<T>>;

#define REP(i, m, n) for (ll i = (m), i##_len = (ll)(n); i < i##_len; ++(i))
#define REPM(i, m, n) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; ++(i))
#define REPR(i, m, n) for (ll i = (m), i##_min = (ll)(n); i >= i##_min; --(i))
#define REPD(i, m, n, d) for (ll i = (m), i##_len = (ll)(n); i < i##_len; i += (d))
#define REPMD(i, m, n, d) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; i += (d))
#define REPI(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define ALL(x) begin(x), end(x)
#define SIZE(x) ((ll)(x).size())
#define PERM(c)                                                                                                        \
    sort(ALL(c));                                                                                                      \
    for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))
#define UNIQ(v) v.erase(unique(ALL(v)), v.end());

#define endl '\n'
#define sqrt sqrtl
#define floor floorl
#define log2 log2l
ll ceil(const ll a, const ll b) { return (a + b - 1) / b; }

constexpr ll INF = 1'010'000'000'000'000'017LL;
constexpr ll MOD = 1'000'000'007LL; // 1e9 + 7
constexpr ld EPS = 1e-12;
constexpr ld PI = 3.14159265358979323846;

template <typename T> istream &operator>>(istream &is, vc<T> &vec) { // vector 入力
    for (T &x : vec) is >> x;
    return is;
}
template <typename T> ostream &operator<<(ostream &os, vc<T> &vec) { // vector 出力 (for dump)
    os << "{";
    REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "" : ", ");
    os << "}";
    return os;
}
template <typename T> ostream &operator>>(ostream &os, vc<T> &vec) { // vector 出力 (inline)
    REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "\n" : " ");
    return os;
}

template <typename T, typename U> istream &operator>>(istream &is, pair<T, U> &pair_var) { // pair 入力
    is >> pair_var.first >> pair_var.second;
    return is;
}
template <typename T, typename U> ostream &operator<<(ostream &os, pair<T, U> &pair_var) { // pair 出力
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

// map, um, set 出力
template <class T> ostream &out_iter(ostream &os, T &map_var) {
    os << "{";
    REPI(itr, map_var) {
        os << *itr;
        auto itrcp = itr;
        if (++itrcp != map_var.end()) os << ", ";
    }
    os << "}";
    return os;
}
template <typename T, typename U> ostream &operator<<(ostream &os, map<T, U> &map_var) { return out_iter(os, map_var); }
template <typename T, typename U> ostream &operator<<(ostream &os, um<T, U> &map_var) { return out_iter(os, map_var); }
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) { return out_iter(os, set_var); }

// dump
#define DUMPOUT cerr
void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) DUMPOUT << ", ";
    dump_func(move(tail)...);
}

// chmax (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>> bool chmax(T &xmax, const U &x, Comp comp = {}) {
    if (comp(xmax, x)) {
        xmax = x;
        return true;
    }
    return false;
}

// chmin (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>> bool chmin(T &xmin, const U &x, Comp comp = {}) {
    if (comp(x, xmin)) {
        xmin = x;
        return true;
    }
    return false;
}

// ローカル用
#define DEBUG_

#ifdef DEBUG_
#define DEB
#define dump(...)                                                                                                      \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                                                                    \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl                                        \
            << "    ",                                                                                                 \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct AtCoderInitialize {
    static constexpr int IOS_PREC = 60;
    static constexpr bool AUTOFLUSH = false;
    AtCoderInitialize() {
        ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
        cout << fixed << setprecision(IOS_PREC);
        if (AUTOFLUSH) cout << unitbuf;
    }
} ATCODER_INITIALIZE;

string yes = "Yes", no = "No";
// string yes = "YES", no = "NO";

/* #endregion */

using Weight = ll;
using Flow = ll;

// エッジ（本来エッジは双方向だが，ここでは単方向で管理）
struct Edge {
    ll src;        // エッジ始点となる頂点
    ll dst;        // エッジ終点となる頂点
    Weight weight; // 重み
    Flow cap;
    Edge() : src(0), dst(0), weight(0) {}
    Edge(ll src, ll dst, Weight weight) : src(src), dst(dst), weight(weight) {}
};
using Node = vc<Edge>;  // 同じ頂点を始点とするエッジ集合
using Graph = vc<Node>; // graph[i] := 頂点 i を始点とするエッジ集合
// using Array = vector<Weight>;
// using Matrix = vector<Array>;

// 双方向のエッジを追加する
void add_edge(Graph &g, ll a, ll b, Weight w = 1) {
    g[a].emplace_back(a, b, w);
    g[b].emplace_back(b, a, w);
}
// 単方向のアークを追加する
void add_arc(Graph &g, ll a, ll b, Weight w = 1) { g[a].emplace_back(a, b, w); }

// Edge 標準出力
ostream &operator<<(ostream &os, Edge &edge) {
    os << "(" << edge.src << " -> " << edge.dst << ", " << edge.weight << ")";
    return os;
}

/* #endregion */

vll scc(const Graph &g) {
    ll n = SIZE(g); // ノード数
    Graph rg(n);    // 逆向きのグラフ
    for (const Node &es : g)
        for (Edge e : es) {
            swap(e.src, e.dst);
            rg[e.src].emplace_back(e);
        }

    vll order;
    order.reserve(n);

    { // dfs 1回目
        vc<bool> visited(n), added(n);
        REP(i, 0, n) {
            if (visited[i]) continue;
            stack<ll> stk;
            stk.push(i);
            while (!stk.empty()) {
                ll cur = stk.top();
                visited[cur] = true;
                bool pushed = false;
                for (const Edge &e : g[cur])
                    if (!visited[e.dst]) {
                        stk.push(e.dst);
                        pushed = true;
                    }
                if (!pushed) { // カレントノードからは未訪問ノードへ到達できない
                    ll t = stk.top();
                    stk.pop(); // 未訪問ノードへ到達できないノードは除いていく
                    if (!added[t]) {
                        added[t] = true;
                        order.push_back(t);
                    }
                }
            }
        }
        reverse(ALL(order));
    }

    vll ret(n, -1);
    { // dfs 2回目
        ll groupnum = 0;
        for (ll &v : order) {
            if (ret[v] != -1) continue;
            stack<ll> stk;
            stk.push(v);
            while (!stk.empty()) {
                ll cur = stk.top();
                stk.pop();
                ret[cur] = groupnum;
                for (Edge &e : rg[cur])
                    if (ret[e.dst] == -1) stk.push(e.dst);
            }
            ++groupnum;
        }
    }
    return ret;
}

// Problem
void solve() {
    ll n, m;
    cin >> n >> m;
    vll a(m), b(m);
    REP(i, 0, m) {
        cin >> a[i] >> b[i];
        a[i]--, b[i]--;
    }

    Graph g(n);
    REP(i, 0, m) add_arc(g, a[i], b[i]);
    vll group = scc(g);

    um<ll, vll> mp;
    REP(i, 0, n) {
        if (mp.find(group[i]) == mp.end()) mp[group[i]] = vll();
        mp[group[i]].emplace_back(i);
    }
    REPI(itr, mp) {
        ll sz = SIZE(itr->second);
        if (sz > 1) {
            // cout << sz << endl;
            // REP(i, 0, sz) cout << itr->second[i] << endl;
            // ぐるっと1周するようなパスが複数存在する場合，すべてを出力するのは駄目
            // ぐるっと1周する閉路を横切るようなパスが存在しないような閉路を出力しないといけない
            // まずはぐるっと1周するパスを探し，それを横切るエッジがあるなら，そのエッジの終点から始点までのパスを探す
            set<ll> st;
            REP(i, 0, sz) st.insert(itr->second[i]);

            ll start = itr->second[0], term = -1;
            while (true) {
                vll visited(n, -1);
                ll visitcnt = 0;
                visited[start] = visitcnt++;

                queue<Node> que; // 「訪問予定」頂点リスト

                que.push(g[start]);
                Graph g2(n);
                vc<pll> shortcut;
                while (!que.empty()) { // bfs
                    Node cur = que.front();
                    que.pop();

                    for (Edge &e : cur) {
                        if (st.find(e.dst) == st.end()) continue;
                        if (visited[e.dst] == 0) {
                            // dump(e.src, e.dst);
                            term = e.src;
                            continue;
                        } else if (visited[e.dst] != -1) {
                            shortcut.emplace_back(e.src, e.dst);
                            continue;
                        }
                        add_arc(g2, e.dst, e.src);
                        visited[e.dst] = visitcnt++;
                        que.push(g[e.dst]);
                    }
                    if (term != -1) break;
                }
                vll nodes = {term};
                set<ll> st2;
                st2.insert(term);
                ll cur = term;
                while (cur != start) {
                    // なるべく番号が若いノードを辿っていく
                    ll num = INF, par = -1;
                    for (Edge &e : g2[cur]) {
                        if (chmin(num, visited[e.dst])) par = e.dst;
                    }
                    nodes.push_back(par);
                    st2.insert(par);
                    cur = par;
                }
                reverse(ALL(nodes));
                // dump(start, term, nodes, shortcut);
                if (SIZE(shortcut) == 0) {
                    cout << SIZE(nodes) << endl;
                    REP(i, 0, SIZE(nodes)) cout << (nodes[i] + 1) << endl;
                    return;
                }
                // ショートカットが存在しても，その両端点が nodes に入っていなければセーフ
                bool safe = true;
                for (pll &sc : shortcut) {
                    if (st2.find(sc.first) != st2.end() && st2.find(sc.second) != st2.end()) {
                        safe = false;
                        break;
                    }
                }
                if (safe) {
                    cout << SIZE(nodes) << endl;
                    REP(i, 0, SIZE(nodes)) cout << (nodes[i] + 1) << endl;
                    return;
                }

                start = shortcut[0].second, term = -1;
            }
        }
    }
    cout << -1 << endl;
}

// entry point
int main() {
    solve();
    return 0;
}


Measured memory_bytes: 512