n! means n × (n − 1) × ... × 3 × 2 × 1

Find the sum of the digits in the number 100!

(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 fact (num)
  (if (= num 1)
      1
      (* num (fact (1- num)))))

(defun euler-20 ()
  (add-digits (fact 100)))