input: Write a function to calculate the sum of the digits in a given number.
  
  assert sum_digits(123) == 6
  assert sum_digits(456) == 15
  assert sum_digits(789) == 24
  assert sum_digits(0) == 0
  assert sum_digits(10) == 1
output: def sum_digits(n):
      return sum(int(digit) for digit in str(n))