input: Write a python function to validate a phone number using regular expression.
  assert validate_phone("404-999-9999")=="Valid phone number"
  assert validate_phone("123-456-7890")=="Valid phone number"
  assert validate_phone("123-45-6789")=="Invalid phone number"
  assert validate_phone("12-3456-7890")=="Invalid phone number"
output: def validate_phone(phone):
      import re
      pattern = re.compile(r'^\d{3}-\d{3}-\d{4}$')
      if pattern.match(phone):
          return "Valid phone number"
      else:
          return "Invalid phone number"