using System;
using System.Collections.Generic;
using System.Linq;

class Solution
{
    static void Main()
    {
        var s = Console.ReadLine();
        var alpha = new Dictionary<char, int>();

        foreach (var c in s)
        {
            if (!alpha.ContainsKey(c))
            {
                alpha[c] = 0;
            }
            alpha[c]++;
        }

        if (alpha.Count(a => a.Value == 2) == 6)
        {
            var orphan = alpha.First(a => a.Value == 1).Key;
            Console.WriteLine(orphan);
        }
        else
        {
            Console.WriteLine("Impossible");
        }

    }
}