code: def find_number(regex, s):
      
      result = find_string(regex, s)
      if result is None:
          return None
      return int(result)
docs: Find a number using a given regular expression.
      If the string cannot be found, returns None.
      The regex should contain one matching group, 
      as only the result of the first group is returned.
      The group should only contain numeric characters ([0-9]+).
      
      s - The string to search.
      regex - A string containing the regular expression.
      
      Returns an integer or None.