#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
#include <stack>
#include <set>
using namespace std;
#define MOD (long long int)(1e9+7)
#define ll long long int
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define reps(i,n) for(int i=1; i<=(int)(n); i++)
#define REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = mpow(a,b/2); return memo*memo%MOD;}
  else return mpow(a,b-1) * a % MOD;
}
ll lpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = lpow(a,b/2); return memo*memo;}
  else return lpow(a,b-1) * a;
}
ll gcd(ll a, ll b){
  if(b==0) return a;
  else return gcd(b, a%b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n){
  if(kaijo_memo.size() > n) return kaijo_memo[n];
  if(kaijo_memo.size() == 0) kaijo_memo.push_back(1);
  while(kaijo_memo.size() <= n) kaijo_memo.push_back(kaijo_memo[kaijo_memo.size()-1] * kaijo_memo.size() % MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n){
  if(gyaku_kaijo_memo.size() > n) return gyaku_kaijo_memo[n];
  if(gyaku_kaijo_memo.size() == 0) gyaku_kaijo_memo.push_back(1);
  while(gyaku_kaijo_memo.size() <= n) gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size()-1] * mpow(gyaku_kaijo_memo.size(), MOD-2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r){
  if(n == r) return 1;//0個の丸と-1個の棒みたいな時に時に効く？不安.
  if(n < r || r < 0) return 0;
  ll ret = 1;
  ret *= kaijo(n); ret %= MOD;
  ret *= gyaku_kaijo(r); ret %= MOD;
  ret *= gyaku_kaijo(n-r); ret %= MOD;
  return ret;
}

vector<int> G[1200];
int checked[1200];

int main(void){
  int n,m;cin>>n>>m;
  rep(i,m){
    int a,b;cin>>a>>b;
    a--;b--;
    G[a].push_back(b);
  }
  vector<int> len;
  rep(node, n){
    queue<int> q;
    q.push(node);
    rep(i,n){
      checked[i] = false;
    }
    bool ok = false;
    int t;
    for(t=0; t<=1005; t++){
      queue<int> q2;
      while(q.size() > 0){
        int now = q.front(); q.pop();
        if(t != 0 && now == node){
          ok = true;
          break;
        }
        if(checked[now]) continue;
        checked[now] = true;
        rep(i, G[now].size()){
          int next = G[now][i];
          q2.push(next);
        }
      }
      if(ok) break;
      q = q2;
    }
    if(t > 1002){
      len.push_back(INF);
    }else{
      len.push_back(t);
    }
    
  }
  int ans_l = INF-10, ans_i = -1;
  rep(i,len.size()){
    if(ans_l > len[i]){
      ans_i = i;
      ans_l = len[i];
    }
  }
  if(ans_l == INF-10){
    cout<<-1<<endl;
    return 0;
  }

  queue<vector<int>> qq;
  qq.push(vector<int>{ans_i});
  while(true){
    vector<int> load = qq.front(); qq.pop();
    int now = load.back();
    rep(i, G[now].size()){
      int next = G[now][i];
      if(next == load[0]){
        cout<<load.size()<<endl;
        rep(j,load.size()){
          cout<<load[j]+1<<endl;
        }
        return 0;
      }else{
        vector<int> nextload;
        rep(j,load.size()){
          nextload.push_back(load[j]);
        }
        nextload.push_back(next);
        qq.push(nextload);
      }
    }
  }

  return 0;
}


Measured memory_bytes: 384