using System;
using System.Linq;

class No587
{
    static void Main()
    {
        var str = Console.ReadLine().OrderBy(x => x).ToArray();
        var cCount = str.Distinct().Count();
        var ans = "Impossible";
        if (cCount == 7)
        {
            for (int i = 0; i < 13; i += 2)
            {
                if (i <= 11 && str[i] == str[i + 2]){
                  ans = "Impossible";
                  break;  
                } 
                if (i == 12&&ans=="Impossible")
                {
                    ans = str.Last().ToString();
                    break;
                }
                if (str[i] != str[i + 1])　ans = str[i].ToString();
            }
        }
        Console.WriteLine(ans);
    }
}