using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace YukiCoder
{
	public class RisingSun
	{
		private static Dictionary<char, int> HoldingTiles = new Dictionary<char, int>()
		{
			{'a',0},
			{'b',0},
			{'c',0},
			{'d',0},
			{'e',0},
			{'f',0},
			{'g',0},
			{'h',0},
			{'i',0},
			{'j',0},
			{'k',0},
			{'l',0},
			{'m',0},
		};

		public static void Main()
		{
			List<char> NowHolding = Console.ReadLine().ToCharArray().ToList<char>();

			foreach(var TargetHolding in NowHolding)
			{
				if (HoldingTiles.ContainsKey(TargetHolding) == false)
					continue;

				HoldingTiles[TargetHolding] = HoldingTiles[TargetHolding] + 1;
			}

			Possible();
		}

		public static void Possible()
		{
			if (HoldingTiles.Count(x => x.Value == 0) > 2)
			{
				Console.WriteLine("Impossible");
				return;
			}

			var NotHaving = HoldingTiles.Where(x => x.Value == 0).FirstOrDefault();

			if(NotHaving.Key != 0)
			{
				Console.WriteLine(NotHaving.Key);
				return;
			}

			foreach(var Tile in HoldingTiles.Keys)
			{
				Console.WriteLine(Tile);
				return;
			}
		}
	
	}
}