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

namespace _0446
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            var alpha = "abcdefghijklm";
            var diff = alpha.Except(s).ToArray();

            if (diff.Count() == 0)
            {
                Console.WriteLine(string.Join("\n", "abcdefghijklm".ToArray()));
            }
            else if (diff.Count() == 1)
            {
                Console.WriteLine(diff);
            }
            else
            {
                Console.WriteLine("Impossible");
            }
        }
    }
}