Euler Project 112

Working from left-to-right if no digit is exceeded by the digit to its left it is called an increasing number; for example, 134468.
Similarly if no digit is exceeded by the digit to its right it is called a decreasing number; for example, 66420.
We shall call a positive integer that is neither increasing nor decreasing a "bouncy" number; for example, 155349.
Clearly there cannot be any bouncy numbers below one-hundred, but just over half of the numbers below one-thousand (525) are bouncy. In fact, the least number for which the proportion of bouncy numbers first reaches 50% is 538.
Surprisingly, bouncy numbers become more and more common and by the time we reach 21780 the proportion of bouncy numbers is equal to 90%.
Find the least number for which the proportion of bouncy numbers is exactly 99%.

My Solution

(defun IsBouncyNumber(n)

  (let* ((numberstr (format nil "~a" n))

          (strlength (- (length numberstr) 2))

          (IsUp NIL)

          (IsDown NIL))

    (loop for i from 0 to strlength

           do(progn (if (> (char-int (elt numberstr i))

                                 (char-int (elt numberstr (+ i 1))))

                        (setq IsDown T)

                        (progn (if (< (char-int (elt numberstr i))

                                       (char-int (elt numberstr (+ i 1))))

                                  (setq IsUp T))))))

    (if (and IsDown IsUp)

        T

        NIL)))

(defun Euler112(n)

  (let ((constnum 99)

         (num 0)

         (currentnum 0))

    (loop for i from 100 to n

            do(progn (if (IsBouncyNumber i)

                              (progn (incf num)

                                        (setq currentnum i)))

                          (if (= 99 (/ (* num 100) i))

                              (return currentnum))))))