// includes {{{
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<tuple>
#include<cmath>
#include<random>
#include<cassert>
#include<bitset>
#include<cstdlib>
// #include<deque>
// #include<multiset>
// #include<cstring>
// #include<bits/stdc++.h>
// }}}
using namespace std;
using ll = long long;

// #undef DEBUG
// #define DEBUG
// DEBUG {{{
#include <array>
#include <deque>
#include <iostream>
#include <list>
#include <queue>
#include <stack>
#include <tuple>
#include <valarray>
#include <vector>
template < int n, class... T >
typename std::enable_if< (n >= sizeof...(T)) >::type __output_tuple(
    std::ostream &, std::tuple< T... > const &) {}
template < int n, class... T >
typename std::enable_if< (n < sizeof...(T)) >::type __output_tuple(
    std::ostream &os, std::tuple< T... > const &t) {
  os << (n == 0 ? "" : ", ") << std::get< n >(t);
  __output_tuple< n + 1 >(os, t);
}
template < class... T >
std::ostream &operator<<(std::ostream &os, std::tuple< T... > const &t) {
  os << "(";
  __output_tuple< 0 >(os, t);
  os << ")";
  return os;
}
template < class T, class U >
std::ostream &operator<<(std::ostream &os, std::pair< T, U > const &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template < class T >
std::ostream &operator<<(std::ostream &os, const std::stack< T > &a) {
  os << "{";
  for(auto tmp = a; tmp.size(); tmp.pop())
    os << (a.size() == tmp.size() ? "" : ", ") << tmp.top();
  os << "}";
  return os;
}
template < class T, class Container, class Compare >
std::ostream &operator<<(std::ostream &os,
    std::priority_queue< T, Container, Compare > a) {
  os << "{ (top) ";
  while(a.size()) os << a.top() << (a.size() == 1 ? "" : ", "), a.pop();
  os << " }";
  return os;
}
template < class T, class Container >
std::ostream &operator<<(std::ostream &os, std::queue< T, Container > a) {
  os << "{ ";
  while(a.size()) os << a.front() << (a.size() == 1 ? "" : ", "), a.pop();
  os << " }";
  return os;
}
#ifdef DEBUG
#if !defined(DEBUG_OUT)
// #define DEBUG_OUT std::cerr
#endif
#define dump(...)                                                                \
  [&]() {                                                                        \
    auto __debug_tap = std::make_tuple(__VA_ARGS__);                             \
    DEBUG_OUT << "[" << __LINE__ << "] " << #__VA_ARGS__ << " = " << __debug_tap \
    << std::endl;                                                      \
  }()
template < class T >
inline void dump2D(T &d, size_t sizey, size_t sizex) {
  for(size_t i = 0; i < sizey; i++) {
    DEBUG_OUT << "\t";
    for(size_t j = 0; j < sizex; j++)
      DEBUG_OUT << d[i][j] << (j + 1 == sizex ? "" : "\t");
    DEBUG_OUT << std::endl;
  }
}
template < class T >
inline void dump1D(T &d, size_t sizey) {
  for(size_t i = 0; i < sizey; i++) {
    DEBUG_OUT << d[i] << (i + 1 == sizey ? "" : " ");
  }
  DEBUG_OUT << std::endl;
}
template <
class T, class = typename std::iterator_traits< decltype(begin(T())) >::value_type,
      class = typename std::enable_if< !std::is_same< T, std::string >::value >::type >
      std::ostream &operator<<(std::ostream &os, const T &a) {
        os << "{";
        for(auto ite = begin(a); ite != end(a); ++ite)
          os << (ite == begin(a) ? "" : ", ") << *ite;
        os << "}";
        return os;
      }
#else
#define dump(...) ((void) 42)
#define dump2D(...) ((void) 42)
#define dump1D(...) ((void) 42)
template <
class T, class = typename std::iterator_traits< decltype(begin(T())) >::value_type,
      class = typename std::enable_if< !std::is_same< T, std::string >::value >::type >
      std::ostream &operator<<(std::ostream &os, const T &a) {
        for(auto ite = begin(a); ite != end(a); ++ite)
          os << (ite == begin(a) ? "" : " ") << *ite;
        return os;
      }
#endif
// }}}

vector<vector<int>> g;

void YES(vector<int> ans) {
  set<int> st(ans.begin(), ans.end());
  map<int, int> mp;
  for(int i = 0; i < ans.size(); i++) mp[ans[i]] = i;
  assert(st.size() == ans.size());
  int next = 0;
  vector<int> ans2;
  for(int x : ans) {
    if(next > mp[x]) continue;
    ans2.push_back(x);
    for(auto j : g[x]) {
      if(st.count(j)) {
        next = max(next, mp[j]);
      }
    }
    st.erase(x);
  }

  ans = ans2;
  for(auto &e: ans) e++;

  cout << ans.size() << endl;
  sort(begin(ans), end(ans));
  cout << ans << endl;
  exit(0);
}

void NO() {
  cout << -1 << endl;
  exit(0);
}

// vector<set<int>> gst;

void dfs(int from, int i, vector<int> &used, vector<int>&v, map<int, int>&st) {
  if(used[i]) return;
  used[i] = 1;
  st[i] = v.size();
  v.push_back(i);
  // dump(i, v, g[i]);
  // dump(st);
  int start = -1;
  for(int j : g[i]) {
    // dump(j, st.count(j));
    if(st.count(j)) {
      start = max(start, st[j]);
    }
    if(used[j]) continue;
  }

  if(start != -1) {
      vector<int> ans;
      bool pick = 0;
      for(int x : v) {
        if(st[x] == start) pick = 1;
        if(pick) ans.push_back(x);
      }
      // dump(ans);
      YES(ans);
  }
  for(int j : g[i]) {
    if(used[j]) continue;
    dfs(from, j, used, v, st);
  }
  v.pop_back();
  st.erase(i);
}

int main() {
  std::ios::sync_with_stdio(false), std::cin.tie(0);
  int n, m;
  cin >> n >> m;
  g.resize(n);

  for(int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    g[a].push_back(b);
    // gst[a].insert(b);
  }
  for(int i = 0; i < n; i++) {
    vector<int> v;
    map<int, int> st;
    vector<int> used(n);
    dfs(i, i, used, v, st);
  }
  NO();
  return 0;
}


Measured memory_bytes: 640