#!/usr/bin/env perl

use strict;
use warnings;

my $s = <>; chomp$s;
my @array;
for my $ch (split//, $s) {
  push @array, $ch;
}
@array = sort {$a cmp $b} @array;
my $one;
for (my $i = 0; $i < @array - 1; $i+=2) {
  if ($array[$i] ne $array[$i + 1]) {
    if ($one) {
      print "Impossible\n";
      exit;
    }
    $one = $array[$i];
    $i--;
  }
  if ($i < @array - 2 && $array[$i] eq $array[$i + 2]) {
    print "Impossible\n";
    exit;
  }
}
if ($one) {
  print $one . "\n";
} else {
  print $array[@array - 1] . "\n";
}