#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
template <typename X> ostream& operator << (ostream& x,const vector<X>& v){for(ll i=0;i<v.size();++i) x<<v[i]<<" ";return x;}
template <typename X> ostream& operator << (ostream& x,const set<X>& v){for(auto it:v) x<<it<<" ";return x;}
template <typename X, typename Y> ostream& operator << (ostream& x, const pair<X,Y>& v) {x<<v.ff<<" "<<v.ss;return x;}
template <typename T, typename S> ostream& operator << (ostream& os, const map<T, S>& v) { for (auto it : v) os << it.first << "=>" << it.second << endl; return os; }
struct pair_hash {inline std::size_t operator()(const std::pair<ll, ll> & v) const {return v.first*31+v.second;}};
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define speed ios_base::sync_with_stdio(false);cin.tie(NULL)
#define pb push_back
#define fr first
#define sc second
#define prnt(m) for(auto it = m.begin() ; it != m.end() ; ++it){cout<<*it<<endl;}
#define tra(it,m) for(auto it=m.begin();it!=m.end();++it)
#define DBG(x) cout<<"# # #     "<<x<<'\n';
#define fri(i,s,n) for(ll i=s;i<n;i++)
#define mod 1000000007LL
#define mod2 998244353
#define vct2 vector<vector<ll>>
#define vct vector<ll>
#define pll pair<ll,ll>
#define rz(n) resize(n)
#define all(v) v.begin(),v.end()
#define mkp make_pair
#define arrin(a,n) for(ll i = 0; i < n ; i++){cin>>a[i];}
#define arrout(a,n) for(ll i = 0;i < n; i++){cout<<a[i]<<" ";}
#define ex(s) cout<<s<<endl,exit(0);
#define inf 1e18
#define five 100005;
#define six 1000005;
#define prllclock cerr<<"Time : "<<1000*(ld)clock()/(ld)CLOCKS_PER_SEC<<"ms\n";
//___________________________________________________________________________________________________________________________________________________________
const int N = 1003;
vector<int> dp[N];
int main()
{
       int n,m;
       cin>>n>>m;
       fri(i,0,m){
              int x,y;
              cin>>x>>y;
              --x;--y;
              dp[x].pb(y);
       }
       vector<int> ans;
       int xp = mod;
       fri(i,0,n){
              queue<int> q;
              bool vis[n];
              memset(vis,false,sizeof vis);
              vis[i] = true;
              int par[n];
              q.push(i);
              bool ff = false;
              int lst;
              //par[i] = i;
              while(!q.empty())
              {
                     int curr = q.front();
                     q.pop();
                     for(int k : dp[curr]){
                            if(k==i){
                                   ff = true;
                                   lst = curr;
                                   break;
                            }
                            if(!vis[k]){
                                   par[k] = curr;
                                   vis[k] = true;
                                   q.push(k);
                            }
                     }
              }
              if(ff){
                     vector<int> xx;
                     int j = lst;
                     while(j != i){
                            xx.pb(j);
                            j = par[j];
                     }
                     xx.pb(i);
                     if(xx.size()<xp){
                            xp = xx.size();
                            ans = xx;
                     }
              }
       }
       if(ans.size()==0){
              cout<<-1<<endl;
              return 0;
       }
       cout<<ans.size()<<endl;
       for(int k : ans){cout<<k+1<<endl;}
}


Measured memory_bytes: 256