#include <vector>
#include <iostream>
#include <cmath>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <fstream>
#include <unistd.h>
#include <string.h>
#include <string>
#include <numeric>
#include <queue>
#include <deque>
#include <sstream>
#include <iomanip>
#include <set>
#include <stack>
#include <cassert>
#include <functional>
#include <random>
#include <thread>
#include <chrono>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<vvvl> vvvvl;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vvb> vvvb;
typedef vector<vvvb> vvvvb;
typedef vector<string> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<char> vc;
typedef vector<vector<char>> vvc;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<vvd> vvvd;
typedef pair<ll, ll> PL;
typedef vector<PL> vpl;
typedef vector<vector<PL>> vvpl;
typedef vector<vvpl> vvvpl;
const int INF = 1001001001;
const ll LINF = 1e17;
const double pi = 3.1415926535897932;
const string endstr = "\n";
#define FOR(i, a, b) for(ll i = (a); i < b; i++)
#define RFOR(i, a, b) for(ll i = (a); i > b; i--)
#define rep(i, n) for(ll i = 0; i < n; i++)
#define rrep(i, n) for(ll i = n-1; i > -1; i--)
#define FORMAP(it, m) for(auto it = m.begin(); it != m.end(); it++)
#define ff first
#define ss second
#define pb push_back
#define epb emplace_back
#define ALL(X) (X).begin(),(X).end()

template <typename T>
T gcd(T a, T b) {
    return (a == 0) ? b : gcd(b%a, a);
}
template <typename T>
T lcm(T a, T b) {
    return a / gcd(a, b) * b;
}

bool p_comp_fs(const PL p1, const PL p2){ return p1.first < p2.first;};
bool p_comp_fg(const PL p1, const PL p2){ return p1.first > p2.first;};
bool p_comp_ss(const PL p1, const PL p2){ return p1.second < p2.second;};
bool p_comp_sg(const PL p1, const PL p2){ return p1.second > p2.second;};
template <typename T>
vector<T> uniquen(vector<T> vec){
    sort(ALL(vec));
    vec.erase(unique(vec.begin(), vec.end()), vec.end());
    return vec;
}

inline ll popcnt(ll x){return __builtin_popcountll((unsigned long long)x);};
template<class T> bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
//friend bool operator<(const stc& lhs, const stc& rhs){
//    return std::tie(lhs.l, lhs.r) < std::tie(rhs.l, rhs.r);
//}

vvl G, GR;
ll N;

vl dfs(ll st){
    vl dist(N, LINF); dist[st] = 0;
    vl prev(N, -1);
    queue<ll> q; q.push(st);
    while (!q.empty()) {
        ll now = q.front(); q.pop();
        for(auto c : G[now]){
            if(dist[c] != LINF) continue;
            prev[c] = now;
            dist[c] = dist[now] + 1;
            q.push(c);
        }
    }
    
    ll mn = LINF; ll tgt = -1;
    for(auto c : GR[st]){
        if(chmin(mn, dist[c])){
            tgt = c;
        }
    }
    if(tgt == -1) return vl(N+2);
    
    vl res; res.pb(tgt);
    while(tgt != st){
        tgt = prev[tgt];
        res.pb(tgt);
    }
    return res;
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    
    ll M; cin >> N >> M;
    G = vvl(N); GR = vvl(N);
    rep(i, M){
        ll a, b; cin >> a >> b; a--; b--;
        G[a].pb(b); GR[b].pb(a);
    }
    
    vl ans(N+2);
    rep(i, N){
        auto path = dfs(i);
        if(path.size() < ans.size()) ans = path;
    }
    
    if(ans.size() == N+2){
        cout << -1 << endstr;
        return 0;
    }
    
    cout << ans.size() << endstr;
    rep(i, ans.size()) cout << ans[i]+1 << endstr;
    return 0;
}


Measured memory_bytes: 384