#include<bits/stdc++.h>

using namespace std;

const int mod = 1e9 + 7;

using int64 = long long;
const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

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); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}

int main() {
  int64 N, M;
  cin >> N >> M;
  vector< int > g[1000];
  for(int i = 0; i < M; i++) {
    int a, b;
    cin >> a >> b;
    --a, --b;
    g[a].emplace_back(b);
  }


  int ret = inf;
  for(int i = 0; i < N; i++) {
    vector< int > pre(N, -1), cost(N, -1);
    queue< int > que;
    for(auto &to : g[i]) {
      pre[to] = 133333;
      cost[to] = 0;
      que.emplace(to);
    }
    while(que.size()) {
      auto idx = que.front();
      que.pop();
      for(auto &to : g[idx]) {
        if(cost[to] != -1) continue;
        cost[to] = cost[idx] + 1;
        pre[to] = idx;
        que.emplace(to);
      }
    }
    if(~cost[i]) {
      chmin(ret, cost[i]);
    }
  }
  if(ret == inf) {
    cout << -1 << endl;
    return 0;
  }
  for(int i = 0; i < N; i++) {

    vector< int > pre(N, -1), cost(N, -1);
    queue< int > que;
    for(auto &to : g[i]) {
      pre[to] = 133333;
      cost[to] = 0;
      que.emplace(to);
    }
    while(que.size()) {
      auto idx = que.front();
      que.pop();
      for(auto &to : g[idx]) {
        if(cost[to] != -1) continue;
        cost[to] = cost[idx] + 1;
        pre[to] = idx;
        que.emplace(to);
      }
    }
    if(cost[i] == ret) {
      vector< int > ord{i};
      int cur = i;
      while(pre[cur] != 133333) {
        ord.emplace_back(pre[cur]);
        cur = pre[cur];
      }
      reverse(begin(ord), end(ord));
      cout << ord.size() << endl;
      for(auto &p : ord) cout << p + 1 << endl;
      return 0;
    }
  }
  cout << -1 << endl;

}



Measured memory_bytes: 384