#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<ll>;
using VV = vector<VI>;
using VS = vector<string>;

// tourist set
template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif
// tourist set end

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 (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define rep(i,b) FOR(i, 0, b)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("YES")
#define p_no() p("NO")
#define pb(s) push_back(s)
#define SZ(x) ((int)(x).size())

void no(){p_no(); exit(0);}
void yes(){p_yes(); exit(0);}

const ll mod = 1e9 + 7;
const ll inf = 1e18;

void vprint(vector<ll> A){
    ll L = A.size();
    FOR(i, 0, L){
        if(i) cout << ' ';
        cout << A[i];
    }
    cout << endl;
}

VV G;
ll d[1000][1000];
ll N, M;

VI get_path(ll i, ll j){
  debug(i, j);

  VI rev(N, -1);
  vector<bool> visited(N, false);
  queue<ll> que;
  que.push(i);
  visited[i] = true;
  while(!que.empty()){
    ll from = que.front(); que.pop();
    for(ll to : G[from]){
      // まだ訪れていない点なら
      if(visited[to]) continue;

      visited[to] = true;
      rev[to] = from;
      que.push(to);
    }
  }
  VI ret;
  ret.push_back(j);
  
  ll now = j;
  while(true){
    ret.push_back(rev[now]);
    now = rev[now];
    if(now==i) return ret;
  }
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    cin >> N >> M;

    G.resize(N);

    rep(i, 1000){
      rep(j, 1000){
        d[i][j] = inf;
      }
    }

    rep(i, M){
      ll a, b; cin >> a >> b;
      a--;
      b--;
      G[a].push_back(b);
      d[a][b] = 1;
    }

    // warshall
    FOR(i, 0, N){ // 経由点
        FOR(j, 0, N){ // 始点
            FOR(k, 0, N){ // 終点
                d[j][k] = min(d[j][k], d[j][i] + d[i][k]);
            }
        }
    }

    ll mi = inf;
    ll min_i, min_j;
    rep(i, N){
      rep(j, N){
        if(mi > d[i][j] + d[j][i]){
          mi = d[i][j] + d[j][i];
          min_i = i;
          min_j = j;
        }
      }
    }

    if(mi==inf){
      p(-1);
      return 0;
    }
    
    VI path0 = get_path(min_i, min_j);
    debug(path0);
    VI path1 = get_path(min_j, min_i);
    debug(path1);
    
    set<ll> se;
    for(ll v : path0) se.insert(v);
    for(ll v : path1) se.insert(v);

    p(se.size());
    for(ll v : se){
      p(v+1);
    }
    
    return 0;
}

Measured memory_bytes: 8192