#ifndef DEBUG
#  define NDEBUG
#endif
#include <iostream>
#include <algorithm>
#include <string>
#include <iterator>
#include <utility>
#include <type_traits>
#include <limits>
#include <cassert>
#include <numeric>
#include <vector>
#include <queue>
#include <set>

using namespace std;

// template {{{
// misc {{{
using Z = long long;
[[maybe_unused]]
constexpr Z INF = numeric_limits<Z>::max() / 3;
// }}}

// debug {{{
template <class T>
using enable_if_sequence_t = enable_if_t<
  is_same<
    typename iterator_traits<
      typename remove_cv_t<remove_reference_t<T>>::iterator
    >::iterator_category,
    random_access_iterator_tag>{}>;
#ifdef NDEBUG
# define dump(x)
# define debug if (0)
template <class T>
inline T&& ix(T&& value) { return value; }
template <class T, class Index, class... Indices, class = enable_if_sequence_t<T>>
inline auto&& ix(T&& container, Index index, Indices... indices) {
  return ix(container[index], indices...);
}
#else
# define dump(x) do { \
    cerr << "\e[33m[dump: " << __LINE__ << "] " << (#x) << " = " << x << "\e[0m" << endl; \
  } while (0);
# define debug
template <class T>
inline auto&& ix_impl([[maybe_unused]] int line, [[maybe_unused]] const char* code, T&& value) {
  return value;
}
template <class T, class Index, class... Indices, class = enable_if_sequence_t<T>>
inline auto&& ix_impl(int line, const char* code, T&& container, Index index, Indices... indices) {
  if (index < 0) {
    cerr << "\e[31mix: negative index at line " << line << ", ix(" << code << ")\e[0m" << endl;
    exit(1);
  }
  int size = container.size();
  if (index >= size) {
    cerr << "\e[31mix: index out of range at line " << line << ", ix(" << code << ")\n"
      << "index = " << index << " whereas size = " << size << "\e[0m" << endl;
    exit(1);
  }
  return ix_impl(line, code, container[index], indices...);
}
#define ix(...) ix_impl(__LINE__, #__VA_ARGS__, __VA_ARGS__)
#endif
// }}}

// IO {{{
template <class T, class U>
ostream& operator<< (ostream& os, pair<T, U> p) {
  os << '(' << p.first << ", " << p.second << ')';
  return os;
}
template <class T, class U>
istream& operator>> (istream& is, pair<T, U>& p) {
  is >> p.first >> p.second;
  return is;
}
template <class T>
false_type is_container_impl(...);
template <class T, class = typename T::iterator>
true_type is_container_impl(T);
template <class T>
struct is_container : decltype(is_container_impl<T>(declval<T>())) {};
template <>
struct is_container <string> : std::false_type {};
template <class Container, class = enable_if_t<is_container<Container>{}>>
istream& operator>> (istream& is, Container& cont) {
  for (auto& v : cont) is >> v;
  return is;
}
template <class Container, class = enable_if_t<is_container<Container>{}>>
ostream& operator<< (ostream& os, Container const& cont) {
  bool first = true;
  constexpr char sep =
    is_container<typename Container::value_type>{} ? '\n' : ' ';
  for (auto const& v : cont) {
    if (!first) {
      os << sep;
    }
    first = false;
    os << v;
  }
  return os;
}
// }}}
// }}}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  Z N, M;
  cin >> N >> M;
  using Graph = vector<vector<Z>>;
  Graph graph(N);
  for (auto i = 0; i < M; i++) {
    Z A, B;
    cin >> A >> B;
    A--; B--;
    ix(graph, A).push_back(B);
  }
  vector<Z> path;
  for (auto i = 0; i < N; i++) {
    vector<Z> prevs(N, -1);
    queue<Z> que;
    que.push(i);
    bool found = false;
    while (!que.empty()) {
      Z v = que.front();
      que.pop();
      for (auto&& next : ix(graph, v)) {
        if (ix(prevs, next) != -1) continue;
        ix(prevs, next) = v;
        if (next == i) {
          found = true;
          break;
        }
        que.push(next);
      }
      if (found) break;
    }
    if (!found) continue;
    path.push_back(i);
    for (Z v = ix(prevs, i); v != i; v = ix(prevs, v))
      path.push_back(v);
    path.push_back(i);
    reverse(begin(path), end(path));
    break;
  }
  if (path.empty()) {
    cout << -1 << endl;
    return 0;
  }
  while (true) {
    dump(path);
    set<Z> vertices(begin(path), end(path));
    vector<vector<Z>> partial(N);
    for (auto&& v : vertices)
      for (auto&& to : ix(graph, v))
        if (vertices.count(to))
          ix(partial, v).push_back(to);
    dump(partial);
    auto blame = find_if(begin(vertices), end(vertices),
        [&] (Z k) { return ix(partial, k).size() > 1; });
    if (blame == end(vertices)) break;
    Z v = *blame;
    auto p = find(begin(path), end(path), v);
    Z nxt = *next(p);
    Z bad = *find_if(begin(ix(partial, v)), end(ix(partial, v)),
            [=] (Z k) { return k != nxt; });
    dump(v);
    dump(nxt);
    dump(bad);
    auto q = find(begin(path), end(path), bad);
    vector<Z> new_path;
    if (p > q) {
      dump("meow");
      for (auto it = q; it != p; ++it)
        new_path.push_back(*it);
      new_path.push_back(*p);
      new_path.push_back(*q);
    }
    else {
      dump("nyarn");
      for (auto&& u : path) {
        new_path.push_back(u);
        if (u == *p) break;
      }
      for (auto it = q; it != end(path); ++it)
        new_path.push_back(*it);
    }
    path.swap(new_path);
  }
  cout << path.size() - 1 << endl;
  for_each(next(begin(path)), end(path),
      [&] (auto&& v) { cout << v + 1 << '\n'; });
}



Measured memory_bytes: 384