input: Write a function to determine the sum of the digits of a given number.
  assert sum_of_digits(6342) == 15
  assert sum_of_digits(123) == 6
  assert sum_of_digits(9876) == 30
  assert sum_of_digits(0) == 0
  assert sum_of_digits(10) == 1
output: def sum_of_digits(n):
      return sum(int(digit) for digit in str(n))