// clang-format off
#include <iostream>
#include <iomanip>
#include <fstream>
#include <sstream>
#include <string>

#include <algorithm>
#include <numeric>
#include <limits>
#include <chrono>
#include <functional>
#include <iterator>
#include <random>
#include <complex>
#include <utility>
#include <type_traits>
#include <tuple>
#include <memory>

#include <vector>
#include <array>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <valarray>

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <csignal>
#include <cstdint>
#include <cstddef>

#ifdef ONLINE_JUDGE
#define NDEBUG
#endif
#include <cassert>
#include <stdexcept>
#include <exception>

// << pair
template <typename T1, typename T2>std::ostream&operator<<(std::ostream&os,const std::pair<T1,T2>&p){return os<<"("<<p.first<<", "<<p.second<<")";}
// << container
template<typename,template<typename...> class...>struct is_instantiation_of:public std::false_type{};
template<template<typename...>class U,template<typename...>class...Us,typename...T>struct is_instantiation_of<U<T...>,U,Us...>:public std::true_type{};
template<typename S,template<typename...>class U,template<typename...>class...Us>struct is_instantiation_of<S,U,Us...>:std::integral_constant<bool,is_instantiation_of<S,Us...>::value>{};

template<typename C>std::ostream&_debug_go(std::ostream&,const C&);
template<typename C,typename=std::enable_if_t<is_instantiation_of<C,std::vector,std::deque,std::set,std::map,std::unordered_set,std::unordered_map,std::multiset,std::multimap,std::unordered_multiset,std::unordered_multimap,std::valarray>::value>>std::ostream&operator<<(std::ostream&os,const C&v){return _debug_go(os,v);}
template<typename T,std::size_t N> std::ostream& operator<<(std::ostream&os,const std::array<T,N>&v){return _debug_go(os,v);}
template<typename C>std::ostream& _debug_go(std::ostream&os,const C& v){bool first=true;os<<"[";for(auto i:v){if(!first)os<<", ";else first=false;os<<i;}return os<<"]";}

struct Debugger{template<typename T>Debugger&operator,(const T&v){std::cerr<<v<<" ";return *this;}} dbg;
#ifndef NDEBUG
#define debug(args...) do {dbg,args; cerr << "\n";} while(0)
#else
#define debug(args...)
#endif

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define ITER(i, s, e) for (auto i((s)); i != (e); ++i)
#define IT(i, c) ITER(i, (c).begin(), (c).end())

using namespace std;

using ui = uint32_t;
using ll = int64_t;
using ull = uint64_t;
template <typename T = int> using Pt = complex<T>;

template <typename T>
T cross(const Pt<T>& a, const Pt<T>& b) { return a.real() * b.imag() - a.imag() * b.real(); }

template <typename T>
bool cmpPt(const Pt<T>& a, const Pt<T>& b) {
  return a.real() < b.real() || (a.real() == b.real() && a.imag() < b.imag());
}
// END TEMPLATE
// clang-format on

vector<int> solve(int N, const vector<vector<int>>& E) {
  vector<int> st, col(N);
  vector<vector<int>> off(N);
  vector<pair<pair<int, int>, set<int>>> back;
  function<void(int)> dfs;
  dfs = [&](int u) {
    if (col[u]) return;
    col[u] = 1;
    st.push_back(u);
    for (int v : E[u]) {
      if (col[v] == 0)
        dfs(v);
      else {
        if (col[v] == 1)
          back.push_back(
              {{u, v}, set<int>(find(st.begin(), st.end(), v), st.end())});
        off[u].push_back(v);
      }
    }
    col[u] = 2;
    st.pop_back();
  };
  REP(u, N) { dfs(u); }
  for (const auto& b : back) {
    int u, v;
    tie(v, u) = b.first;
    bool ok = true;
    for (int w : b.second) {
      for (int x : off[w]) {
        if ((w != v || x != u) && b.second.find(x) != b.second.end()) {
          ok = false;
          break;
        }
      }
      if (!ok) break;
    }
    if (ok) {
      return vector<int>(b.second.begin(), b.second.end());
    }
  }
  return {};
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);

  int N, M;
  cin >> N >> M;
  vector<vector<int>> E(N);
  REP(i, M) {
    int u, v;
    cin >> u >> v;
    --u, --v;
    E[u].push_back(v);
  };

  auto ans = solve(N, E);
  if (ans.empty())
    cout << "-1\n";
  else {
    cout << ans.size() << '\n';
    for (int u : ans) cout << u + 1 << '\n';
  }

  return 0;
}


Measured memory_bytes: 2048