Write a function that takes in a nonnegative integer and sums its digits. (Using floor division and modulo might be helpful here!)

def sum_digits(n):

"""Sum all the digits of n.

>>> sum_digits(10) # 1 + 0 = 1

1

>>> sum_digits(4224) # 4 + 2 + 2 + 4 = 12

12

>>> sum_digits(1234567890)

45 """

"*** YOUR CODE HERE ***"