using System;
using System.Linq;

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