#include <bits/stdc++.h>

using namespace std;

#define int long long
#define FOR(i, a, b) for(ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for(ll i = (a); i > (b); --i)
#define REP(i, n) for(ll i = 0; i < (n); ++i)
#define REPR(i, n) for(ll i = n; i >= 0; i--)
#define FOREACH(x, a) for(auto &(x) : (a))
#define VECCIN(x)                                                              \
    for(auto &youso_ : (x)) cin >> youso_
#define bitcnt __builtin_popcount
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
template <typename T = long long> inline T IN() {
    T x;
    cin >> x;
    return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&... tail) {
    cin >> head;
    CIN(move(tail)...);
}
#define CCIN(...)                                                              \
    char __VA_ARGS__;                                                          \
    CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
    double __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
    ll __VA_ARGS__;                                                            \
    CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
    string __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
    {                                                                          \
        FOREACH(x, v) { cout << x << " "; }                                    \
        cout << "\n";                                                          \
    }
template <typename T = string> inline void eputs(T s) {
    cout << s << "\n";
    exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
    std::fill((T *)array, (T *)(array + N), val);
}
template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef vector<double> VD;

const int INF = 1e9;
const int MOD = 1e9 + 7;
const double PI = atan(1.0) * 4.0;
// const int MOD = 998244353;
const ll LINF = 9e18;
const ll dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[] = {0, 1, 1, 1, 0, -1, -1, -1};

VVL G(1000);
VL hist;
bool used[1000], finished[1000];
ll start = -1;

void dfs(int v) {
    used[v] = true;
    hist.emplace_back(v);
    FOREACH(to, G[v]) {
        if(finished[to]) continue;
        if(used[to]) {
            start = to;
            return;
        }
        dfs(to);
        if(start != -1) return;
    }
    hist.pop_back();
    finished[v] = true;
}

signed main() {
    LCIN(N, M);
    REP(i, M) {
        LCIN(A, B);
        A--, B--;
        G[A].emplace_back(B);
    }
    REP(s, N) {
        if(used[s]) continue;
        dfs(s);
        if(start != -1) break;
    }
    if(start == -1) eputs(-1);
    VL cycle;
    while(hist.size()) {
        ll b = hist.back();
        hist.pop_back();
        cycle.emplace_back(b);
        if(b == start) break;
    }
    reverse(All(cycle));
    while(1) {
        ll from = -1, to = -1;
        VL order(N, -1);
        REP(i, cycle.size()) order[cycle[i]] = i;
        REP(i, cycle.size()) {
            ll v = cycle[i];
            FOREACH(nv, G[v]) {
                if(order[nv] != -1 && nv != cycle[(i + 1) % cycle.size()]) {
                    from = i, to = order[nv];
                }
            }
        }
        if(from == -1) break;
        VL ncycle;
        ncycle.emplace_back(cycle[from]);
        ll now = to;
        while(now != from) {
            ncycle.emplace_back(cycle[now]);
            now = (now + 1) % cycle.size();
        }
        cycle = ncycle;
    }
    cout << cycle.size() << "\n";
    REP(i, cycle.size()) cout << cycle[i] + 1 << "\n";
}

Measured memory_bytes: 384