input: 
  def max_num(numbers: List[int]) -> int:
      """Return the maximum number in the given list.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([-1, -2, -5, -6, -4, -10])
      -1
      """
  
output:     if not numbers:
          return None
      max_value = numbers[0]
      for num in numbers:
          if num > max_value:
              max_value = num
      return max_value