#define _USE_MATH_DEFINES
#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <bitset>
#include <iterator>
#include <valarray>
#include <complex>
#include <utility>
#include <string>
#include <sstream>
#include <tuple>
#include <initializer_list>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <cctype>
#include <cmath>
#include <cassert>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <fstream>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define rep1(i, n) for (int i = 1; i <= (int)(n); ++i)
#define all(c) (c).begin(),(c).end()
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using tii = tuple<int, int>;
using tll = tuple<ll, ll>;
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; }
#ifdef DBG
template<typename T> ostream &operator<<(ostream &os, set<T> const &s) { if (s.size() == 0) return os << "{}"; os << "{"; decltype(s.begin()) iter; for (iter = s.begin(), ++iter; iter != s.end(); ++iter) { --iter; os << *iter << ", "; ++iter; } --iter; os << *iter << '}'; return os; }
template<typename T> ostream &operator<<(ostream &os, multiset<T> const &s) { if (s.size() == 0) return os << "{}"; os << "{"; decltype(s.begin()) iter; for (iter = s.begin(), ++iter; iter != s.end(); ++iter) { --iter; os << *iter << ", "; ++iter; } --iter; os << *iter << '}'; return os; }
template<typename T, typename U> ostream &operator<<(ostream &os, map<T, U> const &mp) { if (mp.size() == 0) return os << "{}"; os << "{" << endl; decltype(mp.begin()) iter; for (iter = mp.begin(), ++iter; iter != mp.end(); ++iter) { --iter; os << "  {" << iter->first << ", " << iter->second << "}," << endl; ++iter; } --iter; os << "  {" << iter->first << ", " << iter->second << "}" << endl << '}'; return os; }
template<typename T> ostream &operator<<(ostream &os, unordered_set<T> const &s) { if (s.size() == 0) return os << "{}"; os << "{"; decltype(s.begin()) iter; for (iter = s.begin(), ++iter; iter != s.end(); ++iter) { --iter; os << *iter << ", "; ++iter; } --iter; os << *iter << '}'; return os; }
template<typename T> ostream &operator<<(ostream &os, unordered_multiset<T> const &s) { if (s.size() == 0) return os << "{}"; os << "{"; decltype(s.begin()) iter; for (iter = s.begin(), ++iter; iter != s.end(); ++iter) { --iter; os << *iter << ", "; ++iter; } --iter; os << *iter << '}'; return os; }
template<typename T, typename U> ostream &operator<<(ostream &os, unordered_map<T, U> const &mp) { if (mp.size() == 0) return os << "{}"; os << "{" << endl; decltype(mp.begin()) iter; for (iter = mp.begin(), ++iter; iter != mp.end(); ++iter) { --iter; os << "  {" << iter->first << ", " << iter->second << "}," << endl; ++iter; } --iter; os << "  {" << iter->first << ", " << iter->second << "}" << endl << '}'; return os; }
template<typename T> ostream &operator<<(ostream &os, vector<T> const &v) { os << '['; rep(i, v.size()-1) os << v[i] << ", "; if (v.size() > 0) os << v[v.size()-1]; cerr << ']'; return os; }
template<typename T, unsigned N, unsigned Last> struct TuplePrinter { static void print(ostream &os, T const& v) { os << std::get<N>(v) << ", "; TuplePrinter<T, N + 1, Last>::print(os, v); } };
template<typename T, unsigned N> struct TuplePrinter<T, N, N> { static void print(ostream &os, T const &v) { os << std::get<N>(v); } };
template<typename... Types> ostream& operator<<(ostream &os, tuple<Types...> const &v) { os << "("; TuplePrinter<tuple<Types...>, 0, sizeof...(Types) - 1>::print(os, v); os << ")"; return os; }
template<typename T> void _dbg(T const &t) { cerr << t << endl; }
template<typename T, typename... Args> void _dbg(T const &t, Args... args) { cerr << t << " "; _dbg(args...); }
template<typename... Args> void __dbg(const char *fn, int l, Args... args) { cerr << "[" << fn << ":" << l << "] "; _dbg(args...); }
#define dbg(...) __dbg(__func__, __LINE__, __VA_ARGS__)
#else
#define dbg(...)
#endif
const ll infty = 1223372036854775807LL;
const double eps = 1e-8;

vi find_shortest_loop(const vvi& g, int start_node) {
    int n = g.size();
    vi res;
    queue<int> q;
    q.push(start_node);
    vi prev(n, -1);
    while (!q.empty()) {
        int node = q.front();
        q.pop();
        for (int to : g[node]) if (prev[to] == -1) {
            if (to == start_node) {
                // loop found
                int cur = node;
                while (cur != start_node) {
                    res.push_back(cur);
                    cur = prev[cur];
                }
                res.push_back(start_node);
                return vi(res.rbegin(), res.rend());
            } else {
                prev[to] = node;
                q.push(to);
            }
        }
    }
    return res;
}

vi shortest_loop(const vvi& g) {
    vi res;
    rep(i, g.size()) {
        auto cur = find_shortest_loop(g, i);
        if (res.size() == 0 || (cur.size() > 0 && cur.size() < res.size())) {
            res = cur;
        }
    }
    return res;
}


void solve() {
    int n, m;
    cin >> n >> m;
    vvi g(n);
    rep(i, m) {
        int a, b;
        cin >> a >> b;
        --a; --b;
        g[a].push_back(b);
    }
    auto loop = shortest_loop(g);
    if (loop.size() == 0) {
        cout << -1 << endl;
    } else {
        cout << loop.size() << endl;
        for (int node : loop) {
            cout << node + 1 << endl;
        }
    }
}

int main() {
#ifdef DBG
    ifstream cin("in.txt");
    std::cin.rdbuf(cin.rdbuf());
    //ofstream cout("out.txt");
    //std::cout.rdbuf(cout.rdbuf());
#else
    ios_base::sync_with_stdio(false);
    std::cin.tie(0);
#endif
    solve();
    return 0;
}


Measured memory_bytes: 384