input: Write a python function to convert a phone number string into its numerical representation according to the given rules.
  assert numerical_representation('2002') == '2-4-6-80-2-4-6-80'
  assert numerical_representation('123') == '12-4-6-83-5-9-7'
  assert numerical_representation('456') == '4-4-4-45-5-2-66-6-8-4'
output: def numerical_representation(phone_number):
      mapping = {
          '2': '2-4-6-8',
          '3': '3-5-9-7',
          '4': '4-4-4-4',
          '5': '5-5-2-6',
          '6': '6-6-8-4',
          '7': '7-7-6-3',
          '8': '8-8-8-8',
          '9': '9-9-9-9',
          '0': '0-2-4-6'
      }
      
      result = '-'.join(mapping[digit] for digit in phone_number)
      return result