2^(15) = 32768 and the sum of its digits is
3 + 2 + 7 + 6 + 8 = 26.
What is the sum of the digits of the number 2^(1000)?

(defun add-digits (num)
  "Returns the sum of the number's digits"
  (let ((sum 0)
          (num-string (write-to-string num)))
    (dotimes (i (length num-string))
      (setf sum (+ sum (digit-char-p (schar num-string i)))))
    sum))

(defun euler-16 ()
  (add-digits (expt 2 1000)))