def main():
    phone_number= input('Please enter a phone number in the format XXX-XXX-XXXX: ')
    x = validNumber(phone_number)
    print x
def validNumber(phone_number):
    for j,k in enumerate(phone_number):
        if j in [3,7]:
            if k != '-':
                phone_number=input('Please enter a valid phone number: ')
            return phone_number
        elif not c.isalnum():
            phone_number=input('Please enter a valid phone number: ')
            return phone_number
    return phone_number