using System;
using System.Linq;

class No587
{
    static void Main()
    {
        var str = Console.ReadLine();
        var count = 0;
        var ans = "";
        while (0 < str.Length)
        {
            var temp = str[0];
            str=str.Remove(0,1);
            if (str.Contains(temp))
            {
                str=str.Replace(temp.ToString(), "");
                count++;
            }
            else ans = temp.ToString();
        }
        Console.WriteLine(count == 6 ? ans : "Impossible");
    }
}