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

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

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