#include <bits/stdc++.h>
#define int long long
#define ll long long
using ull = unsigned long long;
using namespace std;
const int INF = 1ll << 50;
const int MOD = 1e9 + 7;
#define dump(x)                             \
    if (dbg) {                              \
        cerr << #x << " = " << (x) << endl; \
    }
#define overload4(_1, _2, _3, _4, name, ...) name
#define FOR1(n) for (ll i = 0; i < (n); ++i)
#define FOR2(i, n) for (ll i = 0; i < (n); ++i)
#define FOR3(i, a, b) for (ll i = (a); i < (b); ++i)
#define FOR4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define FOR(...) overload4(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define FORR(i, a, b) for (int i = (a); i <= (b); ++i)
#define bit(n, k) ((n >> k) & 1) /*nのk bit目*/
template <class T>
bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return 1;
    } else
        return 0;
}
template <class T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return 1;
    } else
        return 0;
}
void Yes(bool flag = true) {
    if (flag)
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
}
void No(bool flag = true) {
    Yes(!flag);
}
void YES(bool flag = true) {
    if (flag)
        cout << "YES" << endl;
    else
        cout << "NO" << endl;
}
void NO(bool flag = true) {
    YES(!flag);
}
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(), (v).end()
#define SZ(x) ((int)(x).size())
//#define P pair<int, int>
//#define V vector<int>
//#define S set<int>
#define itn int
bool dbg = false;

int N, M;
vector<vector<int>> G;
vector<vector<int>> GG;


void solve() {
    int ans = INF;
    vector<int> V_ans;
    FOR(N) {
        vector<int> V;
        vector<int> dist(N, -1);
        queue<int> que;
        dist[i] = 0;
        que.push(i);
        while (!que.empty()) {
            int idx = que.front();
            que.pop();
            for (auto& e : G[idx]) {
                if (dist[e] != -1) {
                    if (e == i)
                        chmin(ans, dist[idx] + 1);
                } else {
                    dist[e] = dist[idx] + 1;
                    que.push(e);
                }
            }
        }


        int idx = i;
        for (itn step = ans - 1; step >= 0; step--) {
            bool flag = false;
            for (auto& e : GG[idx]) {
                if (dist[e] == step) {
                    idx = e;
                    V.eb(e);
                    flag = true;
                    break;
                }
            }
            if (!flag) {
                break;
            }
        }
        if (V.size() == ans) {
            V_ans = V;
        }
    }
    if (ans == INF)
        cout << -1 << endl;
    else {
        cout << ans << endl;
        reverse(all(V_ans));
        for (auto& e : V_ans)
            cout << e + 1 << endl;
    }
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    cin >> N >> M;
    G.resize(N);
    GG.resize(N);
    FOR(M) {
        int a, b;
        cin >> a >> b;
        a--;
        b--;
        G[a].eb(b);
        GG[b].eb(a);
    }

    solve();
    return 0;
}


Measured memory_bytes: 512