#include<iostream>
#include<fstream>
#include<sstream>
#include<algorithm>
#include<vector>
#include<queue>
#include<map>
#include<set>
#include<string>
#include<cmath>
#include<cstdio>
#include<cassert>

using namespace std;

#define REP(i,m,n) for(int i=(m); i<(int)(n); i++)
#define RREP(i,m,n) for(int i=(int)(n-1); i>=m; i--)
#define rep(i,n) REP(i,0,n)
#define rrep(i,n) RREP(i,0,n)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define aut(r,v) __typeof(v) r = (v)
#define each(it,o) for(aut(it,(o).begin()); it!=(o).end(); ++it)
#define reach(it,o) for(aut(it,(o).rbegin()); it!=(o).rend(); ++it)
#define fi first
#define se second
#define debug(...) {cerr<<"[L"<<__LINE__<<"] "; _debug(__VA_ARGS__);}

template<typename T1, typename T2> ostream& operator<<(ostream& o, const pair<T1, T2>& p)
{return o<<"("<<p.first<<", "<<p.second<<")";}
template<typename T>string join(const vector<T>&v, string del=", ")
{stringstream s;rep(i,v.size())s<<del<<v[i];return s.str().substr(del.size());}
template<typename T>ostream& operator<<(ostream& o, const vector<T>&v)
{if(v.size())o<<"["<<join(v)<<"]";return o;}
template<typename T>ostream& operator<<(ostream& o, const vector<vector<T> >&vv)
{int l=vv.size();if(l){o<<endl;rep(i,l){o<<(i==0?"[ ":",\n  ")<<vv[i]<<(i==l-1?" ]":"");}}return o;}
template<typename T>ostream& operator<<(ostream& o, const set<T>& st)
{vector<T> v(st.begin(),st.end());o<<"{ "<<join(v)<<" }";return o;}
template<typename T1, typename T2>ostream& operator<<(ostream& o, const map<T1, T2>& m)
{each(p,m){o<<(p==m.begin()?"{ ":",\n  ")<<*p<<(p==--m.end()?" }":"");}return o;}
inline void _debug(){cerr<<endl;}
template<class First, class... Rest>
void _debug(const First& first, const Rest&... rest){cerr<<first<<" ";_debug(rest...);}

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1*acos(0.0));
const double INF = 0x3f3f3f3f;
const double INFL = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-9;
const double mod = 1e9 + 7;

inline void finput(string filename) {
    freopen(filename.c_str(), "r", stdin);
}

int n,m;
vi g[1010], rg[1010];
vi d;
pii bfs(int s){
    d = vi(n, INF);
    d[s] = 0;
    queue<int> Q;
    Q.push(s);
    while(!Q.empty()){
        int v = Q.front(); Q.pop();
        for(auto u : g[v]){
            if(d[u] > d[v] + 1){
                d[u] = d[v] + 1;
                Q.push(u);
            }else if(u == s && d[u] != INF){
                return pii(d[v], v);
            }
        }
    }
    return pii(INF, -1);
}

int main(){
    ios_base::sync_with_stdio(0);
    // finput("./input");
    cin >> n >> m;
    rep(i,m){
        int a,b;
        cin >> a >> b;
        a--; b--;
        g[a].push_back(b);
        rg[b].push_back(a);
    }

    int min_d = INF;
    int sta = -1;
    rep(i,n){
        int d = bfs(i).fi;
        if(min_d > d){
            min_d = d;
            sta = i;
        }
    }
    if(min_d == INF){
        cout << -1 << endl;
        return 0;
    }

    pii p = bfs(sta);
    int v = p.se;
    vi ans;
    ans.push_back(v);
    while(1){
        bool update = false;
        for(auto u : rg[v]){
            if(d[u] == d[v] - 1){
                v = u;
                ans.push_back(u);
                update = true;
                break;
            }
        }
        if(v == sta) break;
    }
    cout << ans.size() << endl;
    for(auto v : ans) cout << v+1 << endl;
    return 0;
}

Measured memory_bytes: 3768