input: Write a function to calculate the sum of the digits in a given number. The function should return the sum of the digits.
  assert sum_digits(1234)==10
  assert sum_digits(98765)==35
  assert sum_digits(0)==0
  assert sum_digits(111111)==6
  assert sum_digits(123456789)==45
output: def sum_digits(n):
      return sum(int(digit) for digit in str(n))