using System;

namespace _587
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            int[] n = new int[26];
            for (int i = 0; i < s.Length; i++) n[s[i] - 'a']++;
            int[] x = new int[100];
            char ret = ' ';
            for (int i = 0; i < 26; i++)
            {
                if (n[i] == 1) ret = (char)('a' + i);
                x[n[i]]++;
            }
            Console.WriteLine(x[1] == 1 && x[2] == 6 ? ret.ToString() : "Impossible");
        }
    }
}