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

public class Hello
{
    public static void Main()
    {
        var d = new Dictionary<char, int>();
        var s = Console.ReadLine().Trim();
        for (int i = 0; i < 13; i++)
            if (d.ContainsKey(s[i])) d[s[i]]++;
            else d[s[i]] = 1;
        Console.WriteLine(getAns(d));
    }
    public static string getAns (Dictionary<char,int> d)
    {
        var ng = "Impossible";
        if (d.Count() != 7) return ng;
        var a = d.Count(x => x.Value == 1);
        if (a == 1)
            foreach (var x in d)
                if (x.Value == 1) return x.Key.ToString();
        return ng;
    }
}