<?php
$stem = "abcdefghijklm";
$base = array_keys(count_chars($stem, 1));
$str = trim(fgets(STDIN));
$in = count_chars($str,1);
$keys = array_keys($in);
$values = array_values($in);
$maxFrequency = max($values);

if(max($keys) > 109 || min($keys) < 97 || $maxFrequency > 2) {
    echo "Impossible".PHP_EOL;
    exit;
}

if($maxFrequency === 2) {
    ans(array_diff($base, $keys));
} else {
    ans($base);
}

function ans($list)
{
    foreach($list as $char)
    {
        echo chr($char).PHP_EOL;
    }
}