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

class Program
{
  static void Main()
  {
    solve();
  }
  static int CountChar(string s, char c)
  {
    return s.Length - s.Replace(c.ToString(), "").Length;
  }

  static void solve()
  {
    Scanner scanner = new Scanner();
    string str = scanner.next();
    char ans = ' ';
    for (char c = 'a'; c <= 'z'; c++)
    {
      if (CountChar(str, c) > 2)
      {
        Console.WriteLine("Impossible");
        return;
      }
      if (CountChar(str, c) == 1)
      {
        if (ans != ' ')
        {
          Console.WriteLine("Impossible");
          return;
        }
        else
        {
          ans = c;
        }
      }
    }
    Console.WriteLine(ans);
  }
}

class Scanner
{
  string[] s;
  int i;

  char[] cs = new char[] { ' ' };

  public Scanner()
  {
    s = new string[0];
    i = 0;
  }

  public string next()
  {
    if (i < s.Length) return s[i++];
    string st = Console.ReadLine();
    while (st == "") st = Console.ReadLine();
    s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
    i = 0;
    return next();
  }

  public int nextInt()
  {
    return int.Parse(next());
  }

  public long nextLong()
  {
    return long.Parse(next());
  }

  public double nextDouble()
  {
    return double.Parse(next());
  }

}