(defun count-valleys (n &optional (altitude 0) (total 0))
    (if (= n 0)
        total
        (let ((c (read-char)))
             (if (eql c #\U)
                 (count-valleys (1- n) (1+ altitude) total)
                 (if (= altitude 0)
                     (count-valleys (1- n) -1 (1+ total))
                     (count-valleys (1- n) (1- altitude) total))))))

(format t "~d~%" (count-valleys (read)))