input: Write a function to calculate the sum of the digits of a given number. 
  assert sum_of_digits(6342) == 15
  assert sum_of_digits(123456789) == 45
  assert sum_of_digits(0) == 0
  assert sum_of_digits(999999999) == 81
  assert sum_of_digits(1) == 1
output: def sum_of_digits(n):
      return sum(int(digit) for digit in str(n))