class UserComparator implements Comparator<User> {
    public int compare(User u1, User u2) {
        return u1.number.charAt(0) == u2.number.charAt(0) ? 
                Integer.compare(Integer.parseInt(u1.number.substring(1, u1.number.length())), 
                    Integer.parseInt(u2.number.substring(1, u2.number.length()))) : 
                u1.number.charAt(0) == 'V' ? -1:1;

    }
}