using System;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;
using Debug = System.Diagnostics.Trace;
using SB = System.Text.StringBuilder;
using System.Numerics;
using static System.Math;
using Number = System.Int32;

namespace Program {
	public class Solver {
		Random rnd = new Random();
		public void Solve() {
			var n = ri;
			var m = ri;
			var G = Enumerate(n, x => new List<int>());
			for (int i = 0; i < m; i++) {
				var u = ri - 1; var v = ri - 1;
				G[u].Add(v);
			}
			var min = n + 1;
			var mid = -1;
			for (int i = 0; i < n; i++) {
				var d = Enumerate(n, x => 100000);
				var prev = Enumerate(n, x => -1);

				d[i] = 0;
				var q = new Queue<int>();
				q.Enqueue(i);
				while (q.Any()) {
					var p = q.Dequeue();
					foreach (var t in G[p]) {
						Debug.WriteLine($"{p} -> {t}");
						if (d[t] > d[p] + 1) {
							d[t] = d[p] + 1;
							prev[t] = p;
							q.Enqueue(t);
						}
						else if (t == i && min > d[p] + 1) {
							min = d[p] + 1;
							mid = i;
						}
					}
				}
			}
			if (min > n) {
				Console.WriteLine(-1);
				return;
			}
			for (int i = mid; i <= mid; i++) {
				var d = Enumerate(n, x => 100000);
				var prev = Enumerate(n, x => -1);
				d[i] = 0;
				var q = new Queue<int>();
				q.Enqueue(i);
				while (q.Any()) {
					var p = q.Dequeue();
					foreach (var t in G[p]) {
						Debug.WriteLine($"{p} -> {t}");
						if (d[t] > d[p] + 1) {
							d[t] = d[p] + 1;
							prev[t] = p;
							q.Enqueue(t);
						}
						else if (t == i && min == d[p] + 1) {
							Debug.WriteLine($"{i} {t}");
							var ans = new List<int>();
							ans.Add(p);
							while (ans.Last() != i) {
								Debug.WriteLine(ans.Last());
								ans.Add(prev[ans.Last()]);
							}
							Console.WriteLine(ans.Count);
							foreach (var x in ans)
								Console.WriteLine(x + 1);
							return;
						}
					}
				}
			}
		}
		const long INF = 1L << 60;
		static int[] dx = { -1, 0, 1, 0 };
		static int[] dy = { 0, 1, 0, -1 };
		int ri { get { return sc.Integer(); } }
		long rl { get { return sc.Long(); } }
		double rd { get { return sc.Double(); } }
		string rs { get { return sc.Scan(); } }
		public IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput());

		static T[] Enumerate<T>(int n, Func<int, T> f) {
			var a = new T[n];
			for (int i = 0; i < a.Length; ++i) a[i] = f(i);
			return a;
		}
		static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }
	}
}

#region main
static class Ex {
	static public string AsString(this IEnumerable<char> ie) { return new string(ie.ToArray()); }
	static public string AsJoinedString<T>(this IEnumerable<T> ie, string st = " ") {
		return string.Join(st, ie);
	}
	static public void Main() {
		Console.SetOut(new Program.IO.Printer(Console.OpenStandardOutput()) { AutoFlush = true });
		var solver = new Program.Solver();
		/* 
		var t = new System.Threading.Thread(solver.Solve, 50000000);
		t.Start();
		t.Join();
		//*/
		solver.Solve();
		Console.Out.Flush();
	}
}
#endregion
#region Ex
namespace Program.IO {
	using System.IO;
	using System.Text;
	using System.Globalization;

	public class Printer : StreamWriter {
		public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
		public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
	}

	public class StreamScanner {
		public StreamScanner(Stream stream) { str = stream; }

		public readonly Stream str;
		private readonly byte[] buf = new byte[1024];
		private int len, ptr;
		public bool isEof = false;
		public bool IsEndOfStream { get { return isEof; } }

		private byte read() {
			if (isEof) return 0;
			if (ptr >= len) {
				ptr = 0;
				if ((len = str.Read(buf, 0, 1024)) <= 0) {
					isEof = true;
					return 0;
				}
			}
			return buf[ptr++];
		}

		public char Char() {
			byte b = 0;
			do b = read(); while ((b < 33 || 126 < b) && !isEof);
			return (char)b;
		}
		public string Scan() {
			var sb = new StringBuilder();
			for (var b = Char(); b >= 33 && b <= 126; b = (char)read()) sb.Append(b);
			return sb.ToString();
		}
		public string ScanLine() {
			var sb = new StringBuilder();
			for (var b = Char(); b != '\n' && b != 0; b = (char)read()) if (b != '\r') sb.Append(b);
			return sb.ToString();
		}
		public long Long() { return isEof ? long.MinValue : long.Parse(Scan()); }
		public int Integer() { return isEof ? int.MinValue : int.Parse(Scan()); }
		public double Double() { return isEof ? double.NaN : double.Parse(Scan(), CultureInfo.InvariantCulture); }
	}
}

#endregion


Measured memory_bytes: 19744