#define DEBUG 0
/**
 * File    : F.cpp
 * Author  : Kazune Takahashi
 * Created : 6/5/2020, 7:20:49 AM
 * Powered by Visual Studio Code
 */
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <vector>
// ----- boost -----
#include <boost/rational.hpp>
#include <boost/multiprecision/cpp_int.hpp>
// ----- using directives and manipulations -----
using namespace std;
using boost::rational;
using boost::multiprecision::cpp_int;
using ll = long long;
using ld = long double;
template <typename T>
using max_heap = priority_queue<T>;
template <typename T>
using min_heap = priority_queue<T, vector<T>, greater<T>>;
// ----- constexpr for Mint and Combination -----
constexpr ll MOD{1000000007LL};
// constexpr ll MOD{998244353LL}; // be careful
constexpr ll MAX_SIZE{3000010LL};
// constexpr ll MAX_SIZE{30000010LL}; // if 10^7 is needed
// ----- ch_max and ch_min -----
template <typename T>
void ch_max(T &left, T right)
{
  if (left < right)
  {
    left = right;
  }
}
template <typename T>
void ch_min(T &left, T right)
{
  if (left > right)
  {
    left = right;
  }
}
// ----- Mint -----
template <ll MOD = MOD>
class Mint
{
public:
  ll x;
  Mint() : x{0LL} {}
  Mint(ll x) : x{(x % MOD + MOD) % MOD} {}
  Mint operator-() const { return x ? MOD - x : 0; }
  Mint &operator+=(Mint const &a)
  {
    if ((x += a.x) >= MOD)
    {
      x -= MOD;
    }
    return *this;
  }
  Mint &operator-=(Mint const &a) { return *this += -a; }
  Mint &operator++() { return *this += 1; }
  Mint operator++(int)
  {
    Mint tmp{*this};
    ++*this;
    return tmp;
  }
  Mint &operator--() { return *this -= 1; }
  Mint operator--(int)
  {
    Mint tmp{*this};
    --*this;
    return tmp;
  }
  Mint &operator*=(Mint const &a)
  {
    (x *= a.x) %= MOD;
    return *this;
  }
  Mint &operator/=(Mint const &a)
  {
    Mint b{a};
    return *this *= b.power(MOD - 2);
  }
  Mint operator+(Mint const &a) const { return Mint(*this) += a; }
  Mint operator-(Mint const &a) const { return Mint(*this) -= a; }
  Mint operator*(Mint const &a) const { return Mint(*this) *= a; }
  Mint operator/(Mint const &a) const { return Mint(*this) /= a; }
  bool operator<(Mint const &a) const { return x < a.x; }
  bool operator<=(Mint const &a) const { return x <= a.x; }
  bool operator>(Mint const &a) const { return x > a.x; }
  bool operator>=(Mint const &a) const { return x >= a.x; }
  bool operator==(Mint const &a) const { return x == a.x; }
  bool operator!=(Mint const &a) const { return !(*this == a); }
  Mint power(ll N) const
  {
    if (N == 0)
    {
      return 1;
    }
    else if (N % 2 == 1)
    {
      return *this * power(N - 1);
    }
    else
    {
      Mint half = power(N / 2);
      return half * half;
    }
  }
};
template <ll MOD>
Mint<MOD> operator+(ll lhs, Mint<MOD> const &rhs)
{
  return rhs + lhs;
}
template <ll MOD>
Mint<MOD> operator-(ll lhs, Mint<MOD> const &rhs)
{
  return -rhs + lhs;
}
template <ll MOD>
Mint<MOD> operator*(ll lhs, Mint<MOD> const &rhs)
{
  return rhs * lhs;
}
template <ll MOD>
Mint<MOD> operator/(ll lhs, Mint<MOD> const &rhs)
{
  return Mint<MOD>{lhs} / rhs;
}
template <ll MOD>
istream &operator>>(istream &stream, Mint<MOD> &a)
{
  return stream >> a.x;
}
template <ll MOD>
ostream &operator<<(ostream &stream, Mint<MOD> const &a)
{
  return stream << a.x;
}
// ----- Combination -----
template <ll MOD = MOD, ll MAX_SIZE = MAX_SIZE>
class Combination
{
public:
  vector<Mint<MOD>> inv, fact, factinv;
  Combination() : inv(MAX_SIZE), fact(MAX_SIZE), factinv(MAX_SIZE)
  {
    inv[1] = 1;
    for (auto i = 2LL; i < MAX_SIZE; i++)
    {
      inv[i] = (-inv[MOD % i]) * (MOD / i);
    }
    fact[0] = factinv[0] = 1;
    for (auto i = 1LL; i < MAX_SIZE; i++)
    {
      fact[i] = Mint<MOD>(i) * fact[i - 1];
      factinv[i] = inv[i] * factinv[i - 1];
    }
  }
  Mint<MOD> operator()(int n, int k)
  {
    if (n >= 0 && k >= 0 && n - k >= 0)
    {
      return fact[n] * factinv[k] * factinv[n - k];
    }
    return 0;
  }
  Mint<MOD> catalan(int x, int y)
  {
    return (*this)(x + y, y) - (*this)(x + y, y - 1);
  }
};
// ----- for C++14 -----
using mint = Mint<MOD>;
using combination = Combination<MOD, MAX_SIZE>;
template <typename T>
T gcd(T x, T y) { return y ? gcd(y, x % y) : x; }
template <typename T>
T lcm(T x, T y) { return x / gcd(x, y) * y; }
// ----- for C++17 -----
template <typename T>
int popcount(T x) // C++20
{
  int ans{0};
  while (x != 0)
  {
    ans += x & 1;
    x >>= 1;
  }
  return ans;
}
// ----- frequently used constexpr -----
// constexpr double epsilon{1e-10};
// constexpr ll infty{1'000'000'000'000'010LL}; // or
// constexpr int infty{1'000'000'010};
// constexpr int dx[4] = {1, 0, -1, 0};
// constexpr int dy[4] = {0, 1, 0, -1};
// ----- Yes() and No() -----
void Yes()
{
  cout << "Yes" << endl;
  exit(0);
}
void No()
{
  cout << "-1" << endl;
  exit(0);
}
// ----- main() -----

using Edge = tuple<int, int>;

class Solve
{
  int N, M;
  vector<Edge> E;
  vector<vector<int>> V;

public:
  Solve(int N, int M) : N{N}, M{M}, E(M), V(N)
  {
    for (auto i = 0; i < M; ++i)
    {
      int A, B;
      cin >> A >> B;
      --A;
      --B;
      V[A].push_back(B);
      E[i] = Edge{A, B};
    }
  }

  void flush()
  {
    for (auto const &e : E)
    {
      int A, B;
      tie(A, B) = e;
      auto vertexes{bfs(B, A)};
      if (vertexes.empty())
      {
        continue;
      }
      if (is_single(vertexes))
      {
        cout << vertexes.size() << endl;
        for (auto v : vertexes)
        {
          cout << v + 1 << endl;
        }
        exit(0);
      }
    }
    No();
  }

private:
  bool is_single(set<int> const &S)
  {
    for (auto v : S)
    {
      int cnt{0};
      for (auto w : V[v])
      {
        if (S.find(w) != S.end())
        {
          ++cnt;
        }
      }
      if (cnt != 1)
      {
        return false;
      }
    }
    return true;
  }

  set<int> bfs(int v, int w)
  {
    queue<int> Q;
    Q.push(v);
    vector<int> parent(N, -1);
    parent[v] = -2;
    while (!Q.empty())
    {
      int src{Q.front()};
      Q.pop();
      if (src == w)
      {
        break;
      }
      for (auto dst : V[src])
      {
        if (parent[dst] == -1)
        {
          parent[dst] = src;
          Q.push(dst);
        }
      }
    }
    if (parent[w] == -1)
    {
      return {};
    }
    set<int> ans;
    ans.insert(w);
    while (w != v)
    {
      w = parent[w];
      ans.insert(w);
    }
    return ans;
  }
};

int main()
{
  int N, M;
  cin >> N >> M;
  Solve solve(N, M);
  solve.flush();
}


Measured memory_bytes: 384