// May this submission get accepted!

#pragma GCC optimize ("O3")
#pragma GCC target ("tune=native")
#pragma GCC target ("avx")

#include <bits/stdc++.h>

// 汎用マクロ
#define ALL_OF(x) (x).begin(), (x).end()
#define REP(i,n) for (long long i=0, i##_len=(n); i<i##_len; i++)
#define RANGE(i,is,ie) for (long long i=(is), i##_end=(ie); i<=i##_end; i++)
#define DSRNG(i,is,ie) for (long long i=(is), i##_end=(ie); i>=i##_end; i--)
#define STEP(i, is, ie, step) for (long long i=(is), i##_end=(ie), i##_step = (step); i<=i##_end; i+=i##_step)
#define UNIQUE(v) do { sort((v).begin(), (v).end()); (v).erase(unique((v).begin(), (v).end()), (v).end()); } while (false)
template<class T> bool chmax(T &a, const T &b) {if (a < b) {a = b; return true;} return false; }
template<class T> bool chmin(T &a, const T &b) {if (a > b) {a = b; return true;} return false; }
#define INF 0x7FFFFFFF
#define LINF 0x7FFFFFFFFFFFFFFFLL
#define Yes(q) ((q) ? "Yes" : "No")
#define YES(q) ((q) ? "YES" : "NO")
#define Possible(q) ((q) ? "Possible" : "Impossible")
#define POSSIBLE(q) ((q) ? "POSSIBLE" : "IMPOSSIBLE")
#define DUMP(q) cerr << "[DEBUG] " #q ": " << (q) << " at " __FILE__ ":" << __LINE__ << endl
#define DUMPALL(q) do { cerr << "[DEBUG] " #q ": ["; REP(i, (q).size()) { cerr << (q)[i] << (i == i_len-1 ? "" : ", "); } cerr << "] at " __FILE__ ":" << __LINE__ << endl; } while (false)
template<class T> T gcd(const T &a, const T &b) { return a < b ? gcd(b, a) : b ? gcd(b, a % b) : a; }
template<class T> T lcm(const T &a, const T &b) { return a / gcd(a, b) * b; }

// gcc拡張マクロ
#define popcount __builtin_popcount
#define popcountll __builtin_popcountll

// エイリアス
#define DANCE_ long
#define ROBOT_ unsigned
#define HUMAN_ signed
#define CHOKUDAI_ const
using  ll = DANCE_ HUMAN_ DANCE_;
using ull = DANCE_ ROBOT_ DANCE_;
using cll = DANCE_ DANCE_ CHOKUDAI_;
using  ld = long double;
using namespace std;

// モジュール

// 処理内容
int main() {
    
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    ll n, m; cin >> n >> m;
    vector<ll> a(m), b(m);
    vector<vector<ll>> g(n);
    REP(i, m) {
        cin >> a[i] >> b[i];
        --a[i]; --b[i]; // 0-indexed
        g[a[i]].push_back(b[i]);
    }
    
    vector<ll> ans;
    REP(s, n) {

        vector<ll> from(n, -1);
        deque<ll> q = {s};
        from[s] = s;
        bool okay = false;
        while (q.size()) {
            ll u = q.front(); q.pop_front();
            for (ll v : g[u]) {
                if (from[v] == -1) {
                    q.push_back(v);
                    from[v] = u;
                } else if (v == s) {
                    vector<ll> cand = {u};
                    while (cand.back() != s) {
                        cand.push_back(from[cand.back()]);
                    }
                    if (ans.empty() || ans.size() > cand.size()) {
                        ans = cand;
                    }
                    okay = true;
                    break;
                }
            }
            if (okay) break;
        }

    }

    if (ans.size()) {
        sort(ALL_OF(ans));
        cout << ans.size() << endl;
        REP(i, ans.size()) cout << ans[i]+1 << endl;
    } else {
        cout << -1 << endl;
    }
    
}

Measured memory_bytes: 384