(defun run (memory)
  (loop for pc = 0 then next-pc
        until (minusp pc)
        for a = (aref memory pc)
        for b = (aref memory (+ pc 1))
        for c = (aref memory (+ pc 2))
        for next-pc = (cond ((minusp a)
                             (setf (aref memory b) (char-code (read-char)))
                             (+ pc 3))
                            ((minusp b)
                             (write-char (code-char (aref memory a)))
                             (+ pc 3))
                            ((plusp (setf (aref memory b)
                                          (- (aref memory b) (aref memory a))))
                             (+ pc 3))
                            (t c))))

(defun main ()
  (let ((memory (vector 15 17 -1 17 -1 -1 16 1 -1 16 3 -1 15 15 0 0 -1 72
                        101 108 108 111 44 32 119 111 114 108 100 33 10 0)))
    (run memory)))