#!/usr/bin/env perl

use strict;
use warnings;

my $target = "abcdefghijklm";
my %h;
my $count2 = 0;
my $twoer;
my $s = <>; chomp$s;
for my $c (split//, $s) {
  $h{$c}++;
  if ($h{$c} >= 2) {
    if ($count2 == 1) {
      print "Impossible\n";
      exit;
    }
    $count2 = 1;
    $twoer = $c;
  }
  if ($target !~ /$c/) {
    print "Impossible\n";
    exit;
  }
}

if ($twoer) {
  for my $tch (split//, $target) {
    if (!exists$h{$tch}) {
      print "$tch\n";
      exit;
    }
  }
}
print join("\n", split//,$target)