using System;
using System.Linq;
using System.Collections.Generic;

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            ABC142.F();
        }

        class ABC142
        {
            public static void A()
            {
                var N = Read.Int();
                Console.WriteLine((N + 1) / 2 * 1.0 / N);
            }
            public static void B()
            {
                var N = Read.Int();
                var K = Read.Int();
                var H = Read.IntN();
                Console.WriteLine(H.Count(x => x >= K));
            }
            public static void C()
            {
                var N = Read.Int();
                var A = Read.IntN();
                int[] idx = new int[N];
                for(int i = 0; i < N; ++i)
                {
                    idx[A[i] - 1] = i + 1;
                }
                Console.WriteLine(String.Join(" ", idx));
            }
            public static void D()
            {
                var A = Read.Long();
                var B = Read.Long();
                Console.WriteLine(Util.Factorize(Util.GCD(A, B)).Count + 1);
            }
            public static void E()
            {
                var N = Read.Int();
                var M = Read.Int();
                var A = new long[M];
                var C = new int[M];
                for(int m = 0; m < M; ++m)
                {
                    A[m] = Read.Long();
                    var b = Read.Int();
                    int c = 0;
                    foreach(var ci in Read.IntN())
                    {
                        c += 1 << (ci - 1);
                    }
                    C[m] = c;
                }
                var N2 = 1 << N;
                var dp = Util.Gen(N2, M + 1, (long)1e12);
                for(int i = 0; i < M+1; ++i)
                {
                    dp[0, i] = 0;
                }
                for(int m = 0; m < M; ++m)
                {
                    var c = C[m];
                    var a = A[m];
                    for (int i = N2 - 1; i >= 0; --i)
                    {
                        dp[i, m + 1] = Math.Min(dp[i, m + 1], dp[i, m]);
                        dp[i | c, m + 1] = Math.Min(dp[i | c, m + 1], dp[i, m] + a);
                    }
                }
                Console.WriteLine(dp[N2 - 1, M] == (long)1e12 ? -1 : dp[N2 - 1, M]);
            }
            public static void F()
            {
                var N = Read.Int();
                var M = Read.Int();
                var Adj = Read.AdjacencyList(N, M, true);
                var done1 = new bool[N + 1];
                var done2 = new bool[N + 1];
                var history = new Stack<int>();
                for(int i = 1; i <= N; ++i)
                {
                    if (!done1[i])
                    {
                        F_LoopDetect(Adj, i, done1, done2, history);
                        if (history.Count > 0) { break; }
                    }
                }
                if (history.Count == 0) { Console.WriteLine(-1); return; }
                var last = history.Pop();
                List<int> loop = new List<int>();
                HashSet<int> loopH = new HashSet<int>();
                while (history.Count > 0)
                {
                    var v = history.Pop();
                    loop.Add(v);
                    loopH.Add(v);
                    if (last == v) { break; }
                }
                loop.Reverse();
                while (true)
                {
                    var edgeInLoop = F_IsPure(Adj, loop, loopH);
                    if (edgeInLoop == null)
                    {
                        Console.WriteLine(loop.Count);
                        foreach(var v in loop)
                        {
                            Console.WriteLine(v);
                        }
                        return;
                    }
                    var newLoop = new List<int>();
                    var newLoopH = new HashSet<int>();
                    newLoop.Add(edgeInLoop.Item1);
                    newLoop.Add(edgeInLoop.Item2);
                    newLoopH.Add(edgeInLoop.Item1);
                    newLoopH.Add(edgeInLoop.Item2);
                    for(int i = loop.IndexOf(edgeInLoop.Item2)+1; ; ++i)
                    {
                        var v = loop[i % loop.Count];
                        if (newLoopH.Contains(v)) { break; }
                        newLoop.Add(v);
                        newLoopH.Add(v);
                    }
                    loop = newLoop;
                    loopH = newLoopH;
                }
            }
            static bool F_LoopDetect(Dictionary<int, HashSet<int>> adj, int v, bool[] done1, bool[] done2, Stack<int> history)
            {
                done1[v] = true;
                history.Push(v);
                foreach(var w in adj[v])
                {
                    if (done2[w]) { continue; }
                    if(done1[w] && !done2[w])
                    {
                        history.Push(w);
                        return true;
                    }
                    if (F_LoopDetect(adj, w, done1, done2, history))
                    {
                        return true;
                    }
                }
                history.Pop();
                done2[v] = true;
                return false;
            }
            static Tuple<int,int> F_IsPure(Dictionary<int, HashSet<int>> adj, List<int> loop, HashSet<int> loopH)
            {
                var N = loop.Count;
                for(int i = 0; i < N; ++i)
                {
                    var v = loop[i];
                    var w = loop[(i + 1) % N];
                    var ad = adj[v].ToArray();
                    foreach(var u in ad)
                    {
                        if (!loopH.Contains(u))
                        {
                            adj[v].Remove(u);
                        }
                        else
                        {
                            if (w != u)
                            {
                                return new Tuple<int, int>(v, u);
                            }
                        }
                    }
                }
                return null;
            }
        }

        class Util
        {
            public static long GCD(long a, long b)
            {
                while (b > 0)
                {
                    var c = a % b;
                    a = b;
                    b = c;
                }
                return a;
            }
            public static Dictionary<long, int> Factorize(long x)
            {
                var dict = new Dictionary<long, int>();
                long minp = 2;
                while (x > 1)
                {
                    bool ok = false;
                    for (long p = minp; p * p <= x; ++p)
                    {
                        if (x % p == 0)
                        {
                            if (!dict.ContainsKey(p))
                            {
                                dict.Add(p, 0);
                            }
                            dict[p]++;
                            minp = p;
                            x /= p;
                            ok = true;
                            break;
                        }
                    }
                    if (!ok)
                    {
                        if (!dict.ContainsKey(x)) { dict.Add(x, 0); }
                        dict[x]++;
                        break;
                    }
                }
                return dict;
            }
            public static T[,] Gen<T>(int r, int c, T v)
            {
                var matrix = new T[r, c];
                for (int i = 0; i < r; ++i)
                {
                    for (int j = 0; j < c; ++j)
                    {
                        matrix[i, j] = v;
                    }
                }
                return matrix;
            }
        }

        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M, bool isDirectional = false)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i) { dict.Add(i, new HashSet<int>()); }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN(); dict[ab[0]].Add(ab[1]);
                    if (!isDirectional) { dict[ab[1]].Add(ab[0]); }
                }
                return dict;
            }
            public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
            {
                Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new Dictionary<int, long>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1], ab[2]);
                    dict[ab[1]].Add(ab[0], ab[2]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 13920