using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Linq;

internal partial class Solver {
    public void Run() {
        var n = ni();
        var m = ni();
        var a = new int[m];
        var b = new int[m];
        var adj = Enumerable.Range(0, n)
            .Select(_ => new List<int>())
            .ToArray();
        for (int i = 0; i < m; i++) {
            a[i] = ni() - 1;
            b[i] = ni() - 1;
            adj[a[i]].Add(b[i]);
        }
        var minLoop = Enumerable.Range(0, n)
            .Select(i => GetMinLoop(adj, i))
            .Where(p => p != null)
            .OrderBy(p => p.Count)
            .FirstOrDefault();

        if (minLoop == null) {
            cout.WriteLine("-1");
        } else {
            cout.WriteLine(minLoop.Count);
            minLoop.Sort();
            foreach (var p in minLoop) {
                cout.WriteLine(p + 1);
            }
        }
    }

    private List<int> GetMinLoop(List<int>[] adj, int start) {
        var n = adj.Length;
        var visited = new bool[n];
        var prev = new int[n];
        var qu = new Queue<int>();
        var pre = new Queue<int>();
        foreach (var p in adj[start]) {
            qu.Enqueue(p);
            pre.Enqueue(-1);
        }
        while (qu.Count > 0) {
            var current = qu.Dequeue();
            var prevPos = pre.Dequeue();
            if (visited[current]) continue;
            visited[current] = true;
            prev[current] = prevPos;
            if (current == start) {
                var loopPath = new List<int>();
                while (current != -1) {
                    loopPath.Add(current);
                    current = prev[current];
                }
                return loopPath;
            }
            foreach (var p in adj[current]) {
                qu.Enqueue(p);
                pre.Enqueue(current);
            }
        }

        return null;
    }
}

// PREWRITEN CODE BEGINS FROM HERE
internal partial class Solver : Scanner {
    public static void Main(string[] args) {
#if LOCAL
        new Solver(Console.In, Console.Out).Run();
#else
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver(Console.In, Console.Out).Run();
        Console.Out.Flush();
#endif
    }

    public void RunMulti() {
        while (HasNext()) {
            Run();
        }
    }

#pragma warning disable IDE0052
    private readonly TextReader cin;
    private readonly TextWriter cout;
#pragma warning restore IDE0052

    public Solver(TextReader reader, TextWriter writer)
        : base(reader) {
        cin = reader;
        cout = writer;
    }
    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer) {
    }

#pragma warning disable IDE1006
#pragma warning disable IDE0051
    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private double[] nd(int n) { return NextDoubleArray(n); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
#pragma warning restore IDE1006
#pragma warning restore IDE0051
}

public static class LinqPadExtension {
    public static T Dump<T>(this T obj) {
#if LOCAL
        return LINQPad.Extensions.Dump(obj);
#else
        return obj;
#endif
    }
}
public class Scanner {
    private readonly TextReader Reader;
    private readonly Queue<string> TokenQueue = new Queue<string>();
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    public Scanner()
        : this(Console.In) {
    }

    public Scanner(TextReader reader) {
        Reader = reader;
    }

    public int NextInt() { return int.Parse(Next(), ci); }
    public long NextLong() { return long.Parse(Next(), ci); }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size) {
        var array = new string[size];
        for (var i = 0; i < size; i++) {
            array[i] = Next();
        }

        return array;
    }
    public int[] NextIntArray(int size) {
        var array = new int[size];
        for (var i = 0; i < size; i++) {
            array[i] = NextInt();
        }

        return array;
    }

    public long[] NextLongArray(int size) {
        var array = new long[size];
        for (var i = 0; i < size; i++) {
            array[i] = NextLong();
        }

        return array;
    }

    public double[] NextDoubleArray(int size) {
        var array = new double[size];
        for (var i = 0; i < size; i++) {
            array[i] = NextDouble();
        }

        return array;
    }

    public string Next() {
        if (TokenQueue.Count == 0) {
            if (!StockTokens()) {
                throw new InvalidOperationException();
            }
        }
        return TokenQueue.Dequeue();
    }

    public bool HasNext() {
        if (TokenQueue.Count > 0) {
            return true;
        }

        return StockTokens();
    }

    private static readonly char[] _separator = new[] { ' ', '\t' };
    private bool StockTokens() {
        while (true) {
            var line = Reader.ReadLine();
            if (line == null) {
                return false;
            }

            var tokens = line.Split(_separator, StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0) {
                continue;
            }

            foreach (var token in tokens) {
                TokenQueue.Enqueue(token);
            }

            return true;
        }
    }
}


Measured memory_bytes: 22560