#include <bits/stdc++.h>
#define GET_MACRO(_1,_2,_3,_4,_5,_6,_7,_8,NAME,...) NAME
#define pr(...) cerr<< GET_MACRO(__VA_ARGS__,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__) <<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define prArr(a) {cerr<<(#a)<<"={";int i=0;for(auto t:(a))cerr<<(i++?", ":"")<<t;cerr<<"}"<<endl;}
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){
  return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

Int N;
vector<vector<Int> > G;
vector<P> edge;
vector<Int> bfs(Int S){
  vector<Int> D(edge.size(), INF);
  vector<Int> prev(edge.size(), -1);
  queue<Int> Q;

  for(Int eidx:G[S]){
    Q.push(eidx);
    D[eidx] = 0;
  }

  while(!Q.empty()){
    Int eidx = Q.front(); Q.pop();
    Int from = edge[eidx].first;
    Int pos = edge[eidx].second;
    Int cost = D[eidx];
    //pr(S, from, pos, cost);

    if(cost != 0 && pos == S){
      vector<Int> res;
      Int idx = prev[eidx];
      res.push_back(edge[idx].second+1);
      while(idx != -1){
        res.push_back(edge[idx].first + 1);
        idx = prev[idx];
      }
      return res;
    }
    for(Int neidx: G[pos]){
      Int ncost = cost + 1;
      Int to = edge[neidx].second;
      if(to == from) continue;
      if(D[neidx] <= ncost) continue;
      Q.push(neidx);
      D[neidx] = ncost;
      prev[neidx] = eidx;
    }
  }
  return vector<Int>({});
}

signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int M;
  cin>>N>>M;

  map<P, Int> cnt;
  G.resize(N);
  vector<Int> ans;
  for(Int i=0;i<M;i++){
    Int a, b;
    cin>>a>>b; a--, b--;
    G[a].push_back(edge.size());
    edge.push_back(P(a, b));
    //G[b].push_back(edge.size());
    //edge.push_back(P(b, a));
    cnt[P(a, b)]++;
    if(cnt[P(b, a)] != 0)ans = {a+1, b+1};
  }

  if(!ans.empty()){
    cout<<ans.size()<<endl;
    for(Int a:ans) cout<<a<<endl;
    return 0;
  }

  for(Int i=0;i<N;i++){
    auto t = bfs(i);
    if(!t.empty() && (ans.empty() || ans.size() > t.size())) ans = t;
  }

  if(!ans.empty()){
    cout<<ans.size()<<endl;
    for(Int a:ans) cout<<a<<endl;
  }
  else cout<<-1<<endl;

  return 0;
}


Measured memory_bytes: 764