<?php
$input = readline();
echo middleChar($input);

function middleChar($string) {
    $charLength = strlen($string);
    $middle = intval($charLength / 2);
    if(strlen($string) % 2 == 0) {
    return $string[$middle - 1] . $string[$middle];
    } else {
    return $string[$middle];
    }
}
?>