diff --git a/lib/lib-advent.org b/lib/lib-advent.org
index 14bffc0..6b56dc4 100644
--- a/lib/lib-advent.org
+++ b/lib/lib-advent.org
@@ -46,7 +46,7 @@
   #+begin_src emacs-lisp :results none
     (defun advent/dot (a b)
       "Return the dot product of the two vectors"
-      (-reduce #'+ (-map (lambda (x) (* (car x) (cdr x))) (-zip-pair a b))))
+      (-sum (-map (lambda (x) (* (car x) (cdr x))) (-zip-pair a b))))
 
     (defun advent/neighbour (p dir)
       "Returns the neighbour to P in the direction DIR"