(defun water-on-tower (heights n)
  (let ((subtotal-left (nth n heights))))
  (loop for i from 0 to n do
        (setf subtotal-left (max subtotal-left (nth i heights))))
  (let ((subtotal-right (nth n heights))))
  (loop for i from n to (list-length heights) do
        (setf subtotal-right (max subtotal-right (nth i heights))))
  (min subtotal-left subtotal-right))