#include <bits/stdc++.h>
#define whlie while
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define rep(i,N) for(int i = 0; i < (N); i++)
#define repr(i,N) for(int i = (N) - 1; i >= 0; i--)
#define rep1(i,N) for(int i = 1; i <= (N) ; i++)
#define repr1(i,N) for(int i = (N) ; i > 0 ; i--)
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define vrep(v,it) for(auto it = v.begin(); it != v.end(); it++)
#define vrepr(v,it) for(auto it = v.rbegin(); it != v.rend(); it++)
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) ll __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
using namespace std; void solve();
using ll = long long; using vl = vector<ll>;
using vi = vector<int>; using vvi = vector< vector<int> >;
constexpr int inf = 1001001001;
constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); rep(i,s) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}
#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);cout << endl;} while(0)
#else
  #define trc(...)
  #define trca(...)
  int main(){solve();}
#endif

using P = pair<double,double>; using vp = vector<P>;
constexpr int MOD = /**/ 1000000007; //*/ 998244353;
///////
// ダイクストラ法 計算量 O( log(頂点数N) * 辺の数E)
// s...始点 N...頂点の数 D...最短距離を記録するvector G...重み付きグラフ
template<typename T>
void dijkstra(int s, int N, vector<T>& D, vvi &G){ 
  using P = pair<T,int>; // pair(最短距離,頂点の番号)
  T INF = numeric_limits<T>::max() / 2;
  rep(i,N) D[i] = INF;
  priority_queue<P,vector<P>,greater<P> > Q;
  D[s] = 0;
  Q.emplace(0,s);
  while(!Q.empty()){
    P p = Q.top(); Q.pop();
    int v = p.se;
    if(D[v] < p.fi) continue;
    for(auto &e : G[v]) // G 重み付きグラフ
      if( amin(D[e], D[v] + 1) ) Q.emplace(D[e], e);
  }
}

void solve(){
  ini(N,M);
  vvi g,h;
  vvi gd(N,vi(N,0));
  vvi hd(N,vi(N,0));
  g.resize(N); h.resize(N);
  rep(i,M){
    ini(a,b); a--; b--;
    g[a].pb(b);
    h[b].pb(a);
  }
  rep(i,N) trc(g[i],h[i]);
  rep(idx , N){
    dijkstra<int>(idx,N,gd[idx],g);
    dijkstra<int>(idx,N,hd[idx],h);
  }
  rep(i,N) trc(gd[i],hd[i]);
  int flg = 0 , idx2 = -1;
  rep(i,N){
    rep(j,N){
      if(i == j) continue;
      if(gd[i][j] <= 1e4 && hd[i][j] <= 1e4){
        flg = 1 , idx2 = i; break;
      }
      if(flg == 1) break;
    }
    if(flg == 1) break;
  }
  if(flg == 0) die(-1);
  vi heiro;
  {
    vi ans; int idx = idx2;
    vi used(N , -1);
    do{ 
      trc(idx);
      used[idx] = ans.size();
      int start = idx;
      ans.pb(idx); 
      each(x , g[idx]){
        if(gd[x][idx] <= 1e4){
          idx = x; break;
        }
      }
      trc(idx);
    }while(used[idx] < 0);
    int nn = ans.size() - used[idx];
    //out(nn);
    for(int i = used[idx] ; i < ans.size();  i++)heiro.pb(ans[i]);
  }
  trc(heiro);
  vi ans(N , -1);
  rep(i,sz(heiro)) ans[heiro[i]] = heiro[(i + 1) % sz(heiro)];
  vi used(sz(heiro) , 1);
  flg = 1;
  while(flg){
    trc(ans , heiro , used);
    flg = 0;
    rep(i,sz(heiro)) rep(j,sz(heiro)){
      if(i == j) continue;
      if(used[i] == 0 || used[j] == 0) continue;
      if(ans[heiro[i]] == heiro[j]) continue;
      if(gd[heiro[i]][heiro[j]] == 1){
        ans[heiro[i]] = heiro[j];
        for(int x = (i + 1) % sz(heiro) ;x != j ; x = (x + 1) % sz(heiro)) used[x] = 0;
        flg = 1;
        trc(heiro);
      }
    }
    rep(i,sz(heiro)) rep(j,sz(heiro)){
      if(i == j) continue;
      if(used[i] == 0 || used[j] == 0) continue;
      if( ans[heiro[i]] == heiro[j]) continue;
      if(hd[heiro[j]][heiro[i]] == 1){
        ans[heiro[i]] = heiro[j];
        for(int x = (i + 1) % sz(heiro); x != j ; x = (x + 1) % sz(heiro)) used[x] = 0;
        flg = 1;
        //trc(heiro);
      }
    }
  }
  int start = -1 ;
  for(int i = 0; i <= sz(used); i++) if(used[i]) {start = heiro[i];break;}
  vi nya;
  do{
    nya.pb(start);
    start = ans[start];
  }while(start != nya[0]);
  out(nya.size());
  each(x,nya) out(x+1);  
}

Measured memory_bytes: 8320