#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= int(n); i++)
#define rreps(i, n) for (int i = int(n); i >= 1; i--)
#define repc(i, n) for (int i = 0; i <= int(n); i++)
#define rrepc(i, n) for (int i = int(n); i >= 0; i--)
#define repi(i, a, b) for (int i = int(a); i < int(b); i++)
#define repic(i, a, b) for (int i = int(a); i <= int(b); i++)
#define each(x, y) for (auto &x : y)
#define all(a) (a).begin(), (a).end()
#define bit32(x) (1 << (x))
#define bit64(x) (1ll << (x))

using namespace std;

using i32 = int;
using i64 = long long;
using u64 = unsigned long long;
using f80 = long double;
using vi32 = vector<i32>;
using vi64 = vector<i64>;
using vu64 = vector<u64>;
using vf80 = vector<f80>;
using vstr = vector<string>;

inline void yes() { cout << "Yes" << '\n'; exit(0); }
inline void no() { cout << "No" << '\n'; exit(0); }
inline i64 gcd(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); if (a % b == 0) return b; return gcd(b, a % b); }
inline i64 lcm(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); return a / gcd(a, b) * b; }
inline u64 xorshift() { static u64 x = 88172645463325252ull; x = x ^ (x << 7); return x = x ^ (x >> 9); }
template <typename T> class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T> class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T> inline void amax(T &x, T y) { if (x < y) x = y; }
template <typename T> inline void amin(T &x, T y) { if (x > y) x = y; }
template <typename T> inline T exp(T x, i64 n, T e = 1) { T r = e; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; }
template <typename T> inline T bis(T ok, T ng, function<bool(T)> f, T eps = 1) { while (abs(ok - ng) > eps) { T mi = (ok + ng) / 2; (f(mi) ? ok : ng) = mi; } return ok; }
template <typename T> istream& operator>>(istream &is, vector<T> &v) { each(x, v) is >> x; return is; }
template <typename T> ostream& operator<<(ostream &os, vector<T> &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; }
void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; }

struct UnWeightedGraph {
  struct Edge {
    int to;
    Edge(int to) : to(to) {}
  };
  vector<vector<Edge>> edges;
  int n;
  UnWeightedGraph(int n) : n(n) {
    edges.assign(n, vector<Edge>());
  }
  void add_edge(int v, int u) {
    edges[v].emplace_back(u);
  }
  vector<Edge> &operator[](int x) {
    return edges[x];
  }
};

void solve() {
  int N, M;
  cin >> N >> M;
  UnWeightedGraph g(N);
  rep(i, M) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    g.add_edge(a, b);
  }
  auto bfs = [&](int start) -> int {
    queue<int> q;
    vi32 dist(N, 1e9);
    q.emplace(start);
    dist[start] = 0;
    while (q.size()) {
      int u = q.front(); q.pop();
      each(e, g[u]) {
        if (e.to == start) {
          return dist[u] + 1;
        }
        if (dist[e.to] != 1e9) continue;
        q.emplace(e.to);
        dist[e.to] = dist[u] + 1;
      }
    }
    return -1;
  };
  int mi = 1e9, mi_u;
  rep(i, N) {
    int ret = bfs(i);
    if (ret != -1 && ret < mi) {
      mi = ret;
      mi_u = i;
    }
  }
  if (mi == 1e9) {
    cout << -1 << endl;
    return;
  }
  vi32 history;
  vi32 visited(N, 1e9);
  auto dfs = [&](auto dfs, int u, int d) -> bool {
    visited[u] = d;
    each(e, g[u]) {
      if (e.to == mi_u && d + 1 == mi) {
        return true;
      }
      if (visited[e.to] <= d + 1) {
        continue;
      }
      if (dfs(dfs, e.to, d + 1)) {
        history.push_back(e.to);
        return true;
      }
    }
    return false;
  };
  dfs(dfs, mi_u, 0);
  history.push_back(mi_u);
  sort(all(history));
  cout << mi << endl;
  rep(i, mi) cout << history[i] + 1 << endl;
}


Measured memory_bytes: 384