#include <bits/stdc++.h>
#include <unistd.h>
 
using namespace std;
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.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 (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx[4] = { -1,0, 1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
// #define mp make_pair
//#define endl '\n'


const int N = 1111;
vii G(N);
vi nex(N, -1);
vector<bool> all_used(N);
int start_point = -1;


bool dfs(int now, int par, vector<bool> &sosen) {
  all_used[now] = true;
  sosen[now] = true;
  rep (i, G[now].size()) {
    int ch = G[now][i];
    if (sosen[ch]) {
      start_point = ch;
      nex[now] = ch;
      return true;
    }
    if (all_used[ch]) continue;
    else {
      if (dfs(ch, now, sosen)) {
        if (start_point != -1) {
          nex[now] = ch;
          if (start_point == now) {
            start_point = -1;
          }
        }
        return true;
      }
    }
  }
  sosen[now] = false;
  return false;
}

int main() {
  int n, m;
  cin >> n >> m;
  vi uu(m), vv(m);
  rep (i, m) {
    int u, v;
    cin >> u >> v;
    u--; v--;
    G[u].push_back(v);
    uu[i] = u; vv[i] = v;
  }

  bool is_loop = false;
  int turn = 1;
  vector<bool> sosen(n);
  rep (i, n) {
    if (all_used[i]) continue;
    is_loop = dfs(i, -1, sosen);
    if (is_loop) break;
  }

  if (not is_loop) {
    cout << -1 << endl;
    return 0;
  }

  
  
  //DEBUG_VEC(nex);

  while (true) {
    bool is_update = false;
    rep (i, m) {
      int u = uu[i], v = vv[i];
      if (nex[u] != -1 and nex[v] != -1 and nex[u] != v) {
        nex[u] = v;
        set<int> iki;
        iki.insert(u);
        int now = v;
        while (now != u) {
          iki.insert(now);
          now = nex[now];
        }

        rep (i, n) {
          if (nex[i] != -1 and not iki.count(i)) {
            nex[i] = -1;
          }
        }

        is_update = true;
      }
    }
    if (not is_update) break;
  }

  int cnt = 0;
  rep (i, n) {
    if (nex[i] != -1) cnt++;
  }
  cout << cnt << endl;
  rep (i, n) {
    if (nex[i] != -1) cout << i + 1 << endl;
  }
}


Measured memory_bytes: 512