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

class Program
{
    static void Main()
    {
        string S = Console.ReadLine();
        Dictionary<char, int> dic = new Dictionary<char, int>();
        foreach (var item in S)
        {
            if (dic.ContainsKey(item))
                dic[item]++;
            else
                dic[item] = 1;
        }
        if (dic.Where(x => x.Value > 2).Count() > 0)
        {
            Console.WriteLine("Impossible");
            return;
        }
        if (dic.Where(x => x.Value == 1).Count() == 1)
        {
            Console.WriteLine(dic.Where(x => x.Value == 1).First().Key);
        }
        else
        {
            Console.WriteLine("Impossible");
        }
    }
}