def last_name(s):
    """
    Returns the last name in s
    
    Examples:
        last_name_first('Walker White') returns 'White'
        last_name_first('Walker      White') returns 'White'
    
    Parameter s: a name 'first-name last-name'
    Precondition: s is a string 'first-name last-name' with one or more blanks 
    between the two names.
    """

    start_last = s.rfind(' ')
    return s[start_last+1:]