#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int INF = 1e9;
const ll LINF = 1e18;
inline ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
inline ll lcm(ll a, ll b) { return a / gcd(a, b)*b; }
template<class S,class T> ostream& operator << (ostream& out,const pair<S,T>& o){ out << "(" << o.first << "," << o.second << ")"; return out; }
template<class T> ostream& operator << (ostream& out,const vector<T>& V){ for(int i = 0; i < V.size(); i++){ out << V[i]; if(i!=V.size()-1) out << " ";} return out; }
template<class T> ostream& operator << (ostream& out,const vector<vector<T> >& Mat){ for(int i = 0; i < Mat.size(); i++) { if(i != 0) out << endl; out << Mat[i];} return out; }
template<class S,class T> ostream& operator << (ostream& out,const map<S,T>& mp){ out << "{ "; for(auto it = mp.begin(); it != mp.end(); it++){ out << it->first << ":" << it->second; if(mp.size()-1 != distance(mp.begin(),it)) out << ", "; } out << " }"; return out; }
template<typename T>vector<T> make_v(size_t a){return vector<T>(a);}
template<typename T,typename... Ts>auto make_v(size_t a,Ts... ts){return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));}
template<typename T,typename V> typename enable_if<is_class<T>::value==0>::type fill_v(T &t,const V &v){t=v;}
template<typename T,typename V> typename enable_if<is_class<T>::value!=0>::type fill_v(T &t,const V &v){for(auto &e:t) fill_v(e,v);}
/*
 <url:https://atcoder.jp/contests/abc142/tasks/abc142_f>
 問題文============================================================
 =================================================================
 解説=============================================================
 ================================================================
 */

template<class Type>
Type solve(Type res = Type()){
    // Input
    int N,M; cin >> N >> M;
    vector<int> G[1000];
    for(int i = 0; i < M;i++){
        int A,B; cin >> A >> B;
        A--; B--;
        G[A].push_back(B);
    }
    // Main
    set<int> state;
    for(int s = 0; s < N;s++){
        vector<int> visited(1000);
        vector<int> pre(N);
        queue<int> q;
        q.push(s);
        visited[s] = true;
        while(q.size()){
            int i = q.front(); q.pop();
            for(auto j:G[i]){
                if(j == s){
                    set<int> cand = {i};
                    
                    while(i != s){
                        i = pre[i];
                        cand.insert(i);
                    }
                    if(state.size() == 0 || state.size() > cand.size()){
                        state = cand;
                    }
                    while(q.size()) q.pop();
                    break;
                }
                if(visited[j]) continue;
                visited[j] = 1;
            pre[j] = i;
                q.push(j);
            }
        }
    }
    
    // Output
    if(state.size()){
        cout << state.size() << endl;
        for(auto n:state) cout << n+1 << endl;
    }else{
        cout << -1 << endl;
    }

    return res;
}
int main(void) {
    cin.tie(0); ios_base::sync_with_stdio(false);
    solve(0);
    // cout << fixed << setprecision(15) << solve<ll>() << endl;
    return 0;
}


Measured memory_bytes: 384