def get_campus_number(phone_num):
    """Returns: the on-campus (i.e., 5-digit) 
    version of a 10-digit phone number.

    Returns: str of the form 'X-XXXX'

    phone_num: number with area code
    Precondition: phone_num is a 10
    digit string of only numbers"""

    return phone_num[5]+"-"+phone_num[6:10]