protected function parseMarks($marks)
    {
        $markedLines = [];

        foreach (explode(',', $marks) as $nums) {
            if (strpos($nums, '-') !== false) {
                list($from, $to) = explode('-', $nums);
                $markedLines = array_merge($markedLines, range($from, $to));
            } else {
                $markedLines[] = $nums;
            }
        }

        return implode(' ', $markedLines);
    }