#include <cassert>
#include <algorithm>
#include <array>
#include <bitset>
#include <cctype>
#include <cstdint>
#include <cmath>
#include <complex>
#include <chrono>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
#include <random>
#include <memory>
#include <utility>
#include <limits>
#include <list>

/* template start */
 
#define rep(i, a, b) for (long long i = (a); (i) < (b); (i)++)
#define all(i) i.begin(), i.end()
#define debug(...) std::cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

void debug_out(){std::cerr<<std::endl;}

template<typename Head,typename... Tail>
void debug_out(Head h,Tail... t){
  std::cerr<<" "<<h;
  if(sizeof...(t)>0)std::cout<<" :";
  debug_out(t...);
}
 
template <typename T1, typename T2>
std::ostream& operator<<(std::ostream& os, std::pair<T1, T2> pa) {
  return os << pa.first << " " << pa.second;
}
 
template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}
 
template<typename T1,typename T2>
inline bool chmax(T1& a,T2 b){return a<b && (a=b,true);}
 
template<typename T1,typename T2>
inline bool chmin(T1& a,T2 b){return a>b && (a=b,true);}
 
long long pow_mod(long long a, long long b, long long mod=-1) {
  if(b==0)return 1;
  if ((a == 0)||(mod!=-1&&(a+mod)%mod==0))return 0;
  long long x = 1;
  while (b > 0) {
    if (b & 1)x = (mod!=-1)?(x * a) % mod:x*a;
    a = (mod!=-1)?(a * a) % mod:a*a;
    b >>= 1;
  }
  return x;
}

template <std::uint_fast64_t Modulus> class modint {
  using u64 = std::uint_fast64_t;

public:
  u64 a;

  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};

/* template end */

using ll = long long;
using P=std::pair<long long,long long>;

//非再帰トポロジカルソート 隣接リストを渡す 返り値のサイズがnじゃないならnot DAG
std::vector<ll> topo_sort(std::vector<std::vector<ll>>& graph) {
  ll n = graph.size();
  std::vector<ll> ret, num(n, 0);
  std::queue<ll> que;
  for(ll i=0;i<n;i++){
    for (ll j : graph[i]) num[j]++;
  }
  for (ll i = 0; i < n; i++) {
    if (num[i] == 0) que.push(i);
  }
  while (!que.empty()) {
    ll u = que.front();
    que.pop();
    ret.push_back(u);
    for (ll i : graph[u]) {
      num[i]--;
      if (num[i] == 0) que.push(i);
    }
  }

  return ret;
}

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

  ll n,m;
  std::cin>>n>>m;
 
  std::vector<std::vector<ll>> graph(n),graph_inv(n);
  rep(i,0,m){
    ll a,b;
    std::cin>>a>>b;
    a--;b--;
    graph[a].push_back(b);
    graph_inv[b].push_back(a);
  }

  if(topo_sort(graph).size()==n){
    std::cout<<-1<<"\n";
    return 0;
  }
 
  ll min=std::numeric_limits<ll>::max();
  ll itr=-1;
 
  rep(i,0,n){
    std::vector<ll> d(n,std::numeric_limits<ll>::max());
    std::queue<ll> que;

    for(ll e:graph[i]){
      d[e]=1;
      que.push(e);
    }
 
    while(!que.empty()){
      ll now=que.front();que.pop();
      if(now==i)break;
      for(ll e:graph[now]){
        if(chmin(d[e],d[now]+1))que.push(e);
      }
    }
    if(chmin(min,d[i]))itr=i;
  }
 
  std::cout<<min<<"\n";
 
  std::vector<ll> d(n,std::numeric_limits<ll>::max());
  std::queue<ll> que;
 
  for(ll e:graph[itr]){
    d[e]=1;
    que.push(e);
  }
 
  while(!que.empty()){
    ll now=que.front();que.pop();
 
    if(now==itr)break;
 
    for(ll e:graph[now]){
      if(chmin(d[e],d[now]+1))que.push(e);
    }
  }
 
  std::cout<<itr+1<<"\n";
 
  ll now=itr;min--;
  while(min>0){
    for(ll e:graph_inv[now]){
      if(d[e]==min){
        std::cout<<e+1<<"\n";
        min--;
        now=e;
        break;
      }
    }
  }

  return 0;
}



Measured memory_bytes: 384