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.Count(x => x.Value == 2) == 6) 
            Console.WriteLine(dic.First(x => x.Value == 1).Key);
        else
            Console.WriteLine("Impossible");
    }
}