using System;
using System.Linq;

namespace No587
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var groups = Console.ReadLine().GroupBy(c => c).ToArray();

            if (groups.Count(g => g.Count() == 2) == 6)
            {
                Console.WriteLine(groups.Single(g => g.Count() == 1).Key);
                return;
            }

            Console.WriteLine("Impossible");
        }
    }
}