(defun decrement (n) (- n 1))

(defun percent (n percent)
  (cond
    ((> percent 0) (/ (+ (* n percent) 50) 100))
    (t (/ (- (* n percent) 50) 100))
  )
)

(defun compound-interest (principal rate years)
  (cond
    ((= years 0) principal)
    (t (compound-interest (+ principal (percent principal rate))
                          rate
                          (decrement years)
       )
    )
  )
)