#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long



#if 0
const int MOD = ;

inline int lgput(int a, int b) {
    int ans = 1;
    while(b > 0) {
        if(b & 1) ans = (1LL * ans * a) % MOD;
        b >>= 1;
        a = (1LL * a * a) % MOD;
    }
    return ans;
}

inline void mod(int &x) {
    if(x >= MOD)
        x -= MOD;
}

inline void add(int &x, int y) {
    x += y;
    mod(x);
}

inline void sub(int &x, int y) {
    x += MOD - y;
    mod(x);
}

inline void mul(int &x, int y) {
    x = (1LL * x * y) % MOD;
}

inline int inv(int x) {
    return lgput(x, MOD - 2);
}
#endif

#if 0
int fact[], invfact[];

inline void prec(int n) {
    fact[0] = 1;
    for(int i = 1; i <= n; i++) {
        fact[i] = (1LL * fact[i - 1] * i) % MOD;
    }
    invfact[n] = lgput(fact[n], MOD - 2);
    for(int i = n - 1; i >= 0; i--) {
        invfact[i] = (1LL * invfact[i + 1] * (i + 1)) % MOD;
    }
}

inline int comb(int n, int k) {
    if(n < k) return 0;
    return (1LL * fact[n] * (1LL * invfact[k] * invfact[n - k] % MOD)) % MOD;
}
#endif

using namespace std;



int main() {
#if 0
    ifstream cin("A.in");
    ofstream cout("A.out");
#endif
    int i, n, m;
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    cin >> n >> m;
    vector < vector <int> > g(n + 1), gr(n + 1);
    vector < vector <bool> > edge(n + 1, vector <bool>(n + 1));
    for(i = 1; i <= m; i++) {
        int x, y;
        cin >> x >> y;
        g[x].push_back(y);
        gr[y].push_back(x);
        edge[x][y] = 1;
    }

    vector <int> from(n + 1);
    vector <bool> vis(n + 1);

    auto bfs = [&](int a) -> void {
        queue <int> Q;
        Q.push(a);
        vis[a] = 1;
        while(Q.size()) {
            int nod = Q.front();
            Q.pop();
            for(auto it : g[nod]) {
                if(vis[it] == 0) {
                    from[it] = nod;
                    vis[it] = 1;
                    Q.push(it);
                }
            }
        }
    };

    vector <int> deg(n + 1);

    for(int a = 1; a <= n; a++) {
        for(auto b : g[a]) {
            if(edge[b][a]) {
                cout << 2 << "\n" << a << "\n" << b;
                return 0;
            }
            for(auto c : gr[a]) {
                if(edge[b][c]) {
                    cout << 3 << "\n" << a << "\n" << b << "\n" << c;
                    return 0;
                }
                if(edge[c][b]) continue;

                fill(vis.begin(), vis.end(), 0);
                fill(from.begin(), from.end(), 0);
                vis[a] = 1;
                for(auto it : g[a]) {
                    if(it != b) {
                        vis[it] = 1;
                    }
                }
                for(auto it : gr[a]) {
                    if(it != c) {
                        vis[it] = 1;
                    }
                }
                bfs(b);
                if(vis[c]) {
                    //cerr << a << " " << b << " " << c << "\n";
                    vector <int> sol;
                    int nod = c;
                    do {
                        sol.push_back(nod);
                        nod = from[nod];
                    }while(nod != 0);
                    sol.push_back(a);

                    fill(deg.begin(), deg.end(), 0);
                    for(auto x : sol) {
                        for(auto y : g[x]) {
                            deg[y]++;
                        }
                    }
                    bool ok = 1;
                    for(auto it : sol) {
                        ok &= (deg[it] == 1);
                    }
                    if(ok) {
                        cout << sol.size() << "\n";
                        reverse(sol.begin(), sol.end());
                        for(auto it : sol) {
                            cout << it << "\n";
                        }
                        return 0;
                    }
                }
            }
        }
    }

    cout << -1;

    return 0;
}



Measured memory_bytes: 640