#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
template<class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i,a,n) for(int (i)=(a);(i)<(n);++(i))
#define eFOR(i,a,n) for(int (i)=(a);(i)<=(n);++(i))
#define rFOR(i,a,n) for(int (i)=(n)-1;(i)>=(a);--(i))
#define erFOR(i,a,n) for(int (i)=(n);(i)>=(a);--(i))
#define each(i, a) for(auto &i : a)
#define SORT(i) sort((i).begin(),(i).end())
#define rSORT(i,a) sort((i).begin(),(i).end(),(a))
#define all(i) (i).begin(),(i).end()
#define out(y,x) ((y) < 0 || h <= (y) || (x) < 0 || w <= (x))
#define line cout << "-----------------------------\n" 
#define ENDL(i,n) ((i) == (n) - 1 ? "\n" : " ")
#define stop system("pause") //comment out this on AOJ.
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10;
constexpr ld pi = 3.1415926535897932;
template<class T>inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; }return false; }
template<class T>inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; }return false; }
inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); }
template<class T>inline istream& operator>>(istream& is, vector<T>& v) { for (auto& elemnt : v)is >> elemnt; return is; }
template<class T, class U>inline istream& operator>>(istream& is, pair<T, U>& p) { is >> p.first >> p.second; return is; }
template<class T>inline vector<T> vec(size_t a) { return vector<T>(a); }
template<class T>inline vector<T> defvec(T def, size_t a) { return vector<T>(a, def); }
template<class T, class... Ts>inline auto vec(size_t a, Ts... ts) { return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...)); }
template<class T, class... Ts>inline auto defvec(T def, size_t a, Ts... ts) { return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...)); }

vector<VI> g;
deque<int> d;
vector<bool> used;
bool dfs(int now) {
    d.push_back(now);
    used[now] = true;
    each(to, g[now]) {
        if (used[to]) {
            while (to != d.front()) {
                used[d.front()] = false;
                d.pop_front();
            }
            return true;
        }
        else if (dfs(to))return true;
    }
    d.pop_back();
    used[now] = false;
    return false;
}

void input(int n, int m) {
    random_device rnd;
    mt19937 mt(rnd());
    uniform_int_distribution<> rand(0, n - 1);
    map<pair<int, int>, bool> ma;
    FOR(i, 0, m) {
        int a, b;
        do {
            a = rand(mt), b = rand(mt);
        } while (a == b || ma[{min(a, b), max(a, b)}]);
        cout << a + 1 << " " << b + 1 << "\n";
        g[a].push_back(b);
        ma[{min(a, b), max(a, b)}] = true;
    }
}

int main() {
    init();

    int n, m; cin >> n >> m;
    g.resize(n);
    used.resize(n);
    FOR(i, 0, m) {
        int a, b; cin >> a >> b;
        --a, --b;
        g[a].push_back(b);
    }
    //input(n, m);

    FOR(i, 0, n)if (dfs(i))break;
    if (d.empty())cout << "-1\n";
    else {
        for (int cnt = 0; cnt <= d.size(); ++cnt) {
            int now = d.front();
            d.pop_front();
            each(to, g[now])if (used[to] && to != d.front()) {
                while (to != d.front()) {
                    used[d.front()] = false;
                    d.pop_front();
                }
                cnt = 0;
                break;
            }
            d.push_back(now);
        }

        cout << d.size() << "\n";
        FOR(i, 0, d.size())cout << d[i] + 1 << "\n";
    }
}

Measured memory_bytes: 384