#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define pli pair<ll, int>
#define pil pair<int, ll>
#define pll pair<ll, ll>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
#define endl '\n'
const int inf = 1000000007;
const ll INF = 1e18;
// int mod = 998244353;
int mod = 1000000007;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { if (a < b) { a = b; return 1; } return 0; }
ll gcd(ll a, ll b) { if (b == 0) return a; return gcd(b, a % b); }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
template<typename T, typename U> T pow_(T a, U b) { return b ? pow_(a * a, b / 2) * (b % 2 ? a : 1) : 1; }
ll modpow(ll a, ll b, ll _mod) { return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1; }
template<class T, class U> ostream& operator << (ostream& os, const pair<T, U>& p) { os << p.F << " " << p.S; return os; }
template<class T> ostream& operator << (ostream& os, const vector<T>& vec) { rep(i, sz(vec)) { if (i) os << " "; os << vec[i]; } return os; }
template<class T> ostream& operator << (ostream& os, const set<T>& s) { for (auto e : s) os << e; return os; }
template<typename T> inline istream& operator >> (istream& is, vector<T>& v) { rep(j, sz(v)) is >> v[j]; return is; }
template<class T, class T2> inline void add(T &a, T2 b) { (a += b % mod) %= mod; if (a < 0) a += mod; }
template<class T> void operator += (vector<T>& v, vector<T> v2) { rep(i, sz(v2)) v.eb(v2[i]); }


void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(12); // debuged
  int T;
  T = 1;
  while (T--) solve();
}

vector<pii> runlength(vector<int> v) {
  vector<pii> ret;
  int cnt = 1;
  int n = sz(v);
  rep3(i, 1, n) {
    if (v[i] == v[i - 1]) {
      cnt++;
    } else {
      ret.eb(v[i - 1], cnt);
      cnt = 1;
    }
  }
  ret.eb(v[n - 1], cnt);
  return ret;
}

void solve() {
  int n, m;
  cin >> n >> m;
  vector<vector<int> > G(n);
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    a--; b--;
    G[a].eb(b);
  }
  vector<int> ans(n + 1);
  rep(s, n) {
    // sから各頂点へ最小何回で行けるか
    vector<int> prev(n), d(n, inf);
    d[s] = 0;
    queue<int> q;
    q.emplace(s);
    while (!q.empty()) {
      int v = q.front();
      q.pop();
      for (int nv : G[v]) {
        if (chmin(d[nv], d[v] + 1)) {
          q.emplace(nv);
          prev[nv] = v;
        }
      }
    }
    // サイクルをたどる
    // sへむかう辺を探す. ただし, sからたどり着けてる必要がある
    pii best = pii(inf, -1);
    rep(v, n) {
      if (d[v] == inf) continue;
      for (int nv : G[v]) {
        if (nv == s) {
          chmin(best, pii(d[v], v));
        }
      }
    }
    if (best.F == inf) continue;
    vector<int> cycle;
    int v = best.S;
    while (v != s) {
      cycle.eb(v);
      v = prev[v];
    }
    cycle.eb(s);
    if (sz(cycle) < sz(ans)) {
      reverse(all(cycle));
      // cerr << s << "  " << cycle << endl;
      ans = cycle;
    }
  }
  if (sz(ans) == n + 1) {
    cout << -1 << endl;
    return;
  }
  cout << sz(ans) << endl;
  rep(i, sz(ans)) cout << ans[i] + 1 << endl;
}


Measured memory_bytes: 3692