using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;

namespace A
{
    class StartingPoint
    {
        static void Main(string[] args)
        {
            if (args?.Any() == true)
            {
                var assembly = Assembly.GetEntryAssembly();
                var resourceName = "AtCoderCSharp.In.txt";
                var stream = assembly.GetManifestResourceStream(resourceName);
                var streamReader = new StreamReader(stream, Encoding.UTF8);
                var str = streamReader.ReadToEnd();
                var textReader = new StringReader(str);
                Console.SetIn(textReader);
            }

            try
            {
                foreach (var s in new Program().Solve())
                {
                    Console.WriteLine(s);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                if (args?.Any() != true)
                    throw e;
            }

            if (args?.Any() == true)
                Console.ReadKey();
        }
    }

    class Program
    {
        public List<int> ScanInts(int count) => Enumerable.Range(0, count).Select(_ => ScanInt).ToList();
        public List<int> ScanInts(long count) => ScanInts((int)count);
        public List<long> ScanLongs(int count) => Enumerable.Range(0, count).Select(_ => Scan).ToList();
        public List<long> ScanLongs(long count) => ScanLongs((int)count);
        public int ScanInt => int.Parse(Str);
        public long Scan => long.Parse(Str);
        public double ScanDouble => (double)Scan;

        StringBuilder sb = new StringBuilder();

        public string Str => ScanStr();
        public string ScanStr()
        {
            sb.Clear();

            int r = 0;
            bool? b = null;
            do
            {
                r = Console.Read();
                b = IsBreak(r);
            } while (b == true);

            if (b == null)
                throw new Exception("input error: unexpected end of stream");

            do
            {
                sb.Append((char)r);
                r = Console.Read();
                b = IsBreak(r);
            } while (b == false);

            return sb.ToString();
        }

        bool? IsBreak(int c)
        {
            switch (c)
            {
                case -1:
                    return null;
                case (int)' ':
                case (int)'\n':
                case (int)'\r':
                    return true;
                default:
                    return false;
            }
        }

        public IEnumerable<int> Loop(int count) => Enumerable.Range(0, count);
        public IEnumerable<int> Loop(int from, int to) => Enumerable.Range(from, to - from + 1);

        public IEnumerable<long> Loop(long count) => Loop(0, count - 1);
        public IEnumerable<long> Loop(long from, long to)
        {
            for (long i = from; i <= to; i++)
                yield return i;
        }

        public string YesNo(bool isYes) => isYes ? "Yes" : "No";

        public HashSet<T> ToHashSet<T>(IEnumerable<T> ts) => new HashSet<T>(ts);

        public List<HashSet<int>> edges;

        public HashSet<int> Slv(int i)
        {
            var res = new Dictionary<int, int>();

            var queue = new HashSet<int>() { i };

            while (queue.Count > 0)
            {
                var obj = queue.First();
                queue.Remove(obj);

                var nexts = edges[obj];

                if (nexts.Contains(i))
                {
                    var result = new HashSet<int>();
                    var p = obj;
                    while (p != i)
                    {
                        if (nexts.Contains(p))
                            return null;

                        result.Add(p);
                        p = res[p];
                    }
                    result.Add(i);

                    return result;
                }

                foreach (var x in nexts.Where(xx => xx != i))
                {
                    if (res.ContainsKey(x))
                        return null;

                    res.Add(x, obj);
                    queue.Add(x);
                }
            }

            return null;
        }

        public IEnumerable<string> Solve()
        {
            var n = ScanInt;
            var m = ScanInt;
            edges = Loop(n).Select(x => new HashSet<int>()).ToList();

            foreach (var i in Loop(m))
            {
                var a = ScanInt;
                var b = ScanInt;

                edges[a - 1].Add(b - 1);
            }

            foreach (var i in Loop(n))
            {
                var res = Slv(i);

                if (res == null)
                    continue;

                yield return res.Count.ToString();
                foreach (var x in res)
                    yield return (x + 1).ToString();
                yield break;
            }

            yield return "-1";
        }
    }
}

Measured memory_bytes: 20188