#pragma warning disable IDE0011
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;
using static System.Math;
using static Extentions;

class IO
{
    int idx;
    string[] line;

    public void R(params char[] sep) { line = Console.ReadLine().Split(sep); idx = 0; }
    public string S => line[idx++];
    public string[] Ss => line.Skip(idx++).ToArray();
    public char C => char.Parse(line[idx++]);
    public char[] Cs => line.Skip(idx++).Select(char.Parse).ToArray();
    public int I => int.Parse(line[idx++]);
    public int[] Is => line.Skip(idx++).Select(int.Parse).ToArray();
    public long L => long.Parse(line[idx++]);
    public long[] Ls => line.Skip(idx++).Select(long.Parse).ToArray();
    public double F => double.Parse(line[idx++]);
    public double[] Fs => line.Skip(idx++).Select(double.Parse).ToArray();
    public decimal D => decimal.Parse(line[idx++]);
    public decimal[] Ds => line.Skip(idx++).Select(decimal.Parse).ToArray();
    public BigInteger B => BigInteger.Parse(line[idx++]);
    public BigInteger[] Bs => line.Skip(idx++).Select(BigInteger.Parse).ToArray();

    public void Write(params object[] xs)
    {
        Console.Write(xs.First());
        foreach (var x in xs.Skip(1)) Console.Write(" " + x);
        Console.WriteLine();
    }
}

static class Extentions
{
    public static void ForEach<T>(this IEnumerable<T> seq, Action<T> act)
    {
        foreach (var item in seq) act(item);
    }
}

static class Program
{
    static void Main()
    {
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        Solve(new IO());
        Console.Out.Flush();
    }

    static void Solve(IO io)
    {
        io.R();
        var s = io.S;

        var set = new SortedSet<char>();
        s.ForEach(c => set.Add(c));
        set.SymmetricExceptWith("yukicoder");

        io.Write(set.Last());
    }
}