Language: C++

#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
struct Fast {
    Fast() {
        std::cin.tie(0);
        ios::sync_with_stdio(false);
        cout.precision(20);
    }
} fast;

/* define */
#define FOR(I, X, Y) for (long long(I) = (X); (I) < (Y); (I)++)
#define REP(I, X, Y) for (long long(I) = (Y)-1; (I) >= (X); (I)--)
#define ALL(X) (X).begin(), (X).end()
#define pb push_back
#define COUNT(V, X)                           \
    (upper_bound((V).begin(), (V).end(), X) - \
     lower_bound((V).begin(), (V).end(), X))
#define debug(x) cerr << #x << ':' << x << endl;
#define DEBUG(v)                             \
    {                                        \
        cerr << #v << ':';                   \
        for (auto xv : v) cerr << xv << ' '; \
        cerr << endl;                        \
    }
#define Yes(X) cout << (X ? "Yes" : "No") << endl;
#define YES(X) cout << (X ? "YES" : "NO") << endl;
#define ctoi(C) (C - '0')
#define pow2(x) ((long long)((long long)1 << x))

/* alias */
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vii = vector<vector<int>>;
using vl = vector<long long>;
using vll = vector<vector<long long>>;
using pi = pair<int, int>;
using pl = pair<long long, long long>;
template <typename T>
using PQ = priority_queue<T>;
template <typename T>
using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const long long dx[] = {1, 0, -1, 0};
const long long dy[] = {0, 1, 0, -1};
const long long dx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
const long long dy8[] = {0, 1, 1, 1, 0, -1, -1, -1};
const long long dx9[] = {1, 1, 0, -1, -1, -1, 0, 1, 0};
const long long dy9[] = {0, 1, 1, 1, 0, -1, -1, -1, 0};
const int INF = 1000000007;
const long long LINF = 1000000000000000007;

/* func */
template <typename T1, typename T2>
inline bool chmin(T1 &a, const T2 &b) {
    if (a > b) a = b;
    return a > b;
}
template <typename T1, typename T2>
inline bool chmax(T1 &a, const T2 &b) {
    if (a < b) a = b;
    return a < b;
}
long long max(long long x, int y) {
    return max(x, (long long)y);
}
long long max(int x, long long y) {
    return max((long long)x, y);
}
long long min(long long x, int y) {
    return min(x, (long long)y);
}
long long min(int x, long long y) {
    return min((long long)x, y);
}

/* library */

/* main */

int N, M;
vector<vector<int>> adj;
bool has_edge[1010][1010];
set<int> went_set;

bool dfs(const int &first, int pre, int now, vector<int> &went) {
    for (auto x : went_set) {
        if (x != first && x != pre && has_edge[now][x]) {
            return false;
        }
    }
    for (auto to : adj[now]) {
        if (to == first) {
            vector<int> ans;
            FOR(i, 0, went.size()) {
                if (went[i]) {
                    ans.push_back(i + 1);
                }
            }
            cout << ans.size() << endl;
            for (auto x : ans) {
                cout << x << endl;
            }
            return true;
        }
    }
    for (auto to : adj[now]) {
        went[to] = true;
        went_set.insert(to);
        bool ok = dfs(first, now, to, went);
        if (ok) {
            return true;
        }
        went[to] = false;
        went_set.erase(to);
    }
    return false;
}

signed main() {
    cin >> N >> M;
    adj.resize(N);
    FOR(i, 0, M) {
        int A, B;
        cin >> A >> B;
        A--;
        B--;
        adj[A].push_back(B);
        has_edge[A][B] = true;
        has_edge[B][A] = true;
    }
    vector<int> went(N, 0);
    FOR(i, 0, N) {
        FOR(j, 0, N) {
            went[j] = (int)(i == j);
        }
        went_set.clear();
        went_set.insert(i);
        bool ok = dfs(i, -1, i, went);
        if (ok) {
            return 0;
        }
    }
    cout << -1 << endl;
}

Compiler output:
In file included from /usr/include/c++/14/string:43,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:3:
/usr/include/c++/14/bits/allocator.h: In destructor 'std::_Vector_base<std::vector<int>, std::allocator<std::vector<int> > >::_Vector_impl::~_Vector_impl()':
/usr/include/c++/14/bits/allocator.h:182:7: error: inlining failed in call to 'always_inline' 'std::allocator< <template-parameter-1-1> >::~allocator() noexcept [with _Tp = std::vector<int>]': target specific option mismatch
  182 |       ~allocator() _GLIBCXX_NOTHROW { }
      |       ^
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53:
/usr/include/c++/14/bits/stl_vector.h:132:14: note: called from here
  132 |       struct _Vector_impl
      |              ^~~~~~~~~~~~