using System;
using System.Linq;

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