$string  = trim(fgets(STDIN));
if($string_len = strlen($string)%2 === 0) {
    $length= $string_len = strlen($string)/2;
    $string_arr = str_split($string,$length);
    $i = 0;
    $base_string = $string_arr[0];
    $compare_string = $string_arr[1];
    if($base_string === $compare_string) {
        echo "YES";
    } else {
        echo "NO";
    }
} else {
    echo "NO";
}