diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index 3d37a42..9ea15c4 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -12,4 +12,5 @@
                (:file "day1")
                (:file "day2")
                (:file "day3")
-               (:file "day4")))
+               (:file "day4")
+               (:file "day5")))
diff --git a/day5.lisp b/day5.lisp
new file mode 100644
index 0000000..6700d35
--- /dev/null
+++ b/day5.lisp
@@ -0,0 +1,30 @@
+;;;; day5.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day5-next-unit (string &optional (start 0))
+  (position-if (lambda (c) (not (equal c #\0))) string :start start))
+
+(defun day5-prior-unit (string end)
+  (or (position-if (lambda (c) (not (equal c #\0))) string :from-end t :end end)
+      0))
+
+(defun day5-react! (polymer)
+  (loop
+    :for index1 := 0 :then index2
+    :for index2 := (day5-next-unit polymer (1+ index1))
+    :while index2
+    :when (and (equalp (char polymer index1) (char polymer index2))
+               (not (equal (char polymer index1) (char polymer index2))))
+    :do (setf (char polymer index1) #\0 (char polymer index2) #\0 index2 (day5-prior-unit polymer index1))
+    :finally (return (remove #\0 polymer))))
+
+(defun day5 ()
+  (let ((base-polymer (day5-react! (first (read-puzzlefile "input5.txt")))))
+    (format t "The reacted polymer has still length ~a.~%" (length base-polymer))
+    (format t "With removing the right unit, the shortest possible polymer has length ~a.~%"
+            (loop
+              :for problematic-unit :from (char-code #\a) :upto (char-code #\z)
+              :minimize (length (day5-react! (remove (code-char problematic-unit)
+                                                     base-polymer
+                                                     :test #'equalp)))))))