// includes
#include <bits/stdc++.h>
using namespace std;

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define bit(n) (1LL<<(n))
// functions
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
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> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io{
  fast_io(){ios_base::sync_with_stdio(false); cin.tie(0); cout << fixed << setprecision(20);}
} fast_io_;


vector<int> edges[1010];
vector<int> V;

bool vis[1010];
int dfs(int i){
  vis[i] = true;
  V.pb(i);
  for(auto e: edges[i]){
    if(vis[e]){
      return e;
    }else{
      int b = dfs(e);
      if(b != -1)return b;
    }
  }
  vis[i] = false;
  V.pop_back();
  return -1;
}

vector<int> ed[1010];
int n;

vector<int> calc(vector<int> v){
  vector<int> u = v;
  rep(i, sz(v))u.pb(v[i]);
  rep(i, sz(v)){
    int curr = i;
    vector<int> res;
    vector<bool> used(n, false);
    vector<bool> ng(n, false);
    bool ok = true;
    while(ok && curr < i + sz(v)){
      if(ng[u[curr]]){
        ok = false;
        break;
      }
      res.pb(u[curr]);
      used[u[curr]] = true;
      if(sz(ed[u[curr]]) == 1){
        curr++;
        continue;
      }
      bool ex = false;
      for(auto e: ed[u[curr]]){
        if(used[e] && e != u[i]){
          ok = false;
          break;
        }else if(used[e] && e == u[i]){
          ex = true;
        }else if(!used[e]){
          if(curr < sz(u) - 1 && u[curr+1] != e){
            ng[e] = true;
          }
        }
      }
      if(ex){
        break;
      }
      curr++;
    }
    if(ok)return res;
  }
  return v;
}

int main(int argc, char const* argv[])
{
  int m; cin >> n >> m;
  rep(i, m){
    int a, b;
    cin >> a >> b; a--, b--;
    edges[a].pb(b);
  }
  rep(i, n){
    rep(j, n)vis[j] = false;
    V.clear();
    int b = dfs(i);
    if(b != -1){
      vector<bool> cl(n, false);
      vector<int> vt;
      bool ex = false;
      rep(j, sz(V)){
        if(V[j] == b){
          ex = true;
        }
        if(ex)vt.pb(V[j]);
      }
      swap(V, vt);
      rep(j, sz(V))cl[V[j]] = true;
      rep(j, sz(V)){
        for(auto e: edges[V[j]]){
          if(cl[e])ed[V[j]].pb(e);
        }
      }
      auto res = calc(V);
      rep(j, sz(res))res[j]++;
      cout << sz(res) << endl;
      rep(j, sz(res))cout << res[j] << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}


Measured memory_bytes: 512