#include <bits/stdc++.h>
using namespace std;
/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<typename T> using gpp_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename L> using gpp_map = tree<T, L, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T> using gpp_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;*/
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin);i<(end);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) cout << #v << "=" << endl; REP(i_debug, v.size()){ cout << v[i_debug] << ","; } cout << endl;
#define mdebug(m) cout << #m << "=" << endl; REP(i_debug, m.size()){ REP(j_debug, m[i_debug].size()){ cout << m[i_debug][j_debug] << ","; } cout << endl;}
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, vector<T> &v){ for(int i = 0; i < v.size(); i++) { cout << v[i]; if(i != v.size() - 1) cout << endl; }; return os; }
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};
//char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
//const int mod = 998244353;
//#define Add(x, y) x = (x + (y)) % mod
//#define Mult(x, y) x = (x * (y)) % mod

struct edge{int to, cost, id;};

class Graph
{
public:
    int N;
    vector<vector<edge>> G;

    Graph(int N): N(N){
        G = vector<vector<edge>>(N, vector<edge>(0));
    }

    void add_Directed_edge(int from, int to, int cost = 1, int id = 0){
        G[from].push_back(edge({to, cost, id}));
    }

    void add_Undirected_edge(int v1, int v2, int cost = 1, int id = 0){
        add_Directed_edge(v1, v2, cost, id);
        add_Directed_edge(v2, v1, cost, id);
    }

    void erase_edge(int from, int to){
        vec e(0);
        REP(i, G[from].size()){
            if(G[from][i].to == to) e.push_back(i);
        }
        IREP(i, e.size()) G[from].erase(G[from].begin() + e[i]);
    }
    vec dijkstra(int s, int t){
        vec d(N);
        //priority_queue<Pii, vector<Pii>, greater<Pii>> que;
        queue<Pii> que;
        fill(d.begin(), d.end(), INF);
        d[s] = 0;
        que.push(Pii(0, s));

        while(!que.empty()){
            Pii p = que.front(); que.pop();
            int v = p.second;
            if(v == t) return d;
            if(d[v] < p.first) continue;
            REP(i, G[v].size()){
                edge e = G[v][i];
                if(d[e.to] > d[v] + e.cost){
                    d[e.to] = d[v] + e.cost;
                    que.push(Pii(d[e.to], e.to));
                }
            }
        }
        return d;
    }

};


signed main(){

    int N, M; cin >> N >> M;
    Graph G(N), rG(N);
    vec A(M), B(M);
    REP(i, M){
        cin >> A[i] >> B[i];
        A[i]--; B[i]--;
        G.add_Directed_edge(A[i], B[i], 1);
        rG.add_Directed_edge(B[i], A[i], 1);
    }

    vec r;
    REP(i, M){
        G.erase_edge(A[i], B[i]);
        vec d = G.dijkstra(B[i], A[i]);
        if(d[A[i]] < INF){
            rG.erase_edge(B[i], A[i]);
            vec rd = rG.dijkstra(A[i], B[i]);
            int now = B[i];
            do{
                r.pb(now);
                for(auto e: G.G[now]) if(rd[e.to] == rd[now] - 1){
                    now = e.to;
                    break;
                }
            } while(now != A[i]);
            r.pb(A[i]);
            break;
        }
        G.add_Directed_edge(A[i], B[i]);
    }
    if(SZ(r) == 0){
        Out(-1);
        return 0;
    }
    //vdebug(r);

    vec next(N);
    bools exist(N);
    while(true){
        bool f = true;
        fill(all(exist), false);
        REP(i, SZ(r)){
            exist[r[i]] = true;
            next[r[i]] = r[(i + 1) % SZ(r)];
        }

        REP(i, M){
            if(exist[A[i]] && exist[B[i]] && next[A[i]] != B[i]){
                f = false;
                vec r2({A[i], B[i]});
                int now = B[i];
                while(true){
                    now = next[now];
                    if(now == A[i]) break;
                    r2.pb(now);
                }
                r.swap(r2);
                break;
            }
        }

        if(f) break;
    }
    Out(SZ(r));
    REP(i, SZ(r)) cout << r[i] + 1 << endl;

    return 0;
}


Measured memory_bytes: 512