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

namespace YukiCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var S = Console.ReadLine().GroupBy(c=>c);
            if (S.Max(c => c.Count()) == 2)
            {
                var a = S.First(c => c.Count() == 1).ToArray();
                Console.WriteLine(a);
            }
            else Console.WriteLine("Impossible");
            Console.WriteLine();
            Console.ReadLine();
        }
    }
}