using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Text;
using static System.Console;
using static System.Convert;
using static System.Math;
using static Extentions;

class IO
{
    int idx;
    string[] input;

    public IO(TextReader reader)
    {
        input = reader.ReadToEnd().Split(new[] { " ", "\n", "\r" },
            StringSplitOptions.RemoveEmptyEntries);
    }

    T Get<T>(Func<string, T> parser) => parser(input[idx++]);

    public string S => Get(s => s);
    public char C => Get(char.Parse);
    public int I => Get(int.Parse);
    public long L => Get(long.Parse);
    public double F => Get(double.Parse);
    public decimal D => Get(decimal.Parse);
    public BigInteger B => Get(BigInteger.Parse);

    T[] Gets<T>(int n, Func<string, T> parser)
        => input.Skip((idx += n) - n).Take(n).Select(parser).ToArray();

    public string[] Ss(int n) => Gets(n, s => s);
    public char[] Cs(int n) => Gets(n, char.Parse);
    public int[] Is(int n) => Gets(n, int.Parse);
    public long[] Ls(int n) => Gets(n, long.Parse);
    public double[] Fs(int n) => Gets(n, double.Parse);
    public decimal[] Ds(int n) => Gets(n, decimal.Parse);
    public BigInteger[] Bs(int n) => Gets(n, BigInteger.Parse);

    public void Write<T>(params T[] xs) => WriteLine(string.Join(" ", xs));
    public void Write(params object[] xs) => WriteLine(string.Join(" ", xs));
}

static class Extentions
{
}

static class Program
{
    public static void Main()
    {
        var sw = new StreamWriter(OpenStandardOutput()) { NewLine = "\n" };
#if DEBUG
        sw.AutoFlush = true;
#else
        sw.AutoFlush = false;
#endif
        SetOut(sw);
        Solve(new IO(In));
        Out.Flush();
    }

    public static void Solve(IO io)
    {
        const string str = "abcdefghijklm";
        var s = io.S;

        var cnt = new int[13];

        for (var i = 0; i < 13; i++)
        {
            if (!str.Contains(s[i]))
            {
                io.Write("Impossible");
                return;
            }
            cnt[s[i] - 'a']++;
        }

        if (cnt.All(x => x == 1))
        {
            foreach (var c in Enumerable.Range('a', str.Length))
                io.Write((char)c);
            return;
        }

        var idx = Array.FindIndex(cnt, x => x == 0);
        io.Write(str[idx]);
    }
}