input: Write a function to calculate the sum of the digits in a given number. If the given number is negative then return -1.
  def sum_digits(num):
      if num < 0:
          return -1
      total = 0
      while num > 0:
          digit = num % 10
          total += digit
          num = num // 10
      return total
  
  assert sum_digits(123)==6
  assert sum_digits(987)==24
  assert sum_digits(-456)==-1
  assert sum_digits(0)==0
  assert sum_digits(7)==7
output: def sum_digits(num):
      if num < 0:
          return -1
      total = 0
      while num > 0:
          digit = num % 10
          total += digit
          num = num // 10
      return total