input: Write a function to calculate the sum of the digits of a given number. If the number is not positive, return 0.
  def sum_of_digits(number):
      if number <= 0:
          return 0
      total = 0
      for ch in str(number):
          total += int(ch)
      return total
  
  assert sum_of_digits(6342) == 15
  assert sum_of_digits(123) == 6
  assert sum_of_digits(0) == 0
  assert sum_of_digits(-123) == 0
  assert sum_of_digits(987654321) == 45
output: def sum_of_digits(number):
      if number <= 0:
          return 0
      total = 0
      for ch in str(number):
          total += int(ch)
      return total