diff --git a/aoc20.lisp b/aoc20.lisp
index 30cb708..f68d5f3 100644
--- a/aoc20.lisp
+++ b/aoc20.lisp
@@ -176,3 +176,39 @@
   (let ((data (read-4 file)))
     (time (progn (print (%solve-4 data *passport-rules-broken*))
 		 (print (%solve-4 data *passport-rules*))))))
+
+(defun binarize (string bits)
+  (parse-integer (substitute
+		  #\1 (elt bits 1)
+		  (substitute
+		   #\0 (elt bits 0) string))
+		 :radix 2))
+
+(defun read-5 (file)
+  (with-open-file (stream file)
+    (loop for line = (read-line stream nil)
+	  while line
+	  collect (map 'list #'binarize (list (subseq line 0 7) (subseq line 7)) '("FB" "LR")))))
+
+(defun boarding-ids (data)
+  (map 'list (lambda (seat) (+ (* 8 (first seat)) (second seat))) data))
+
+(defun solve-5-1 (data)
+  (first (sort (boarding-ids data) #'>)))
+
+(defvar p1-answer)
+
+(defun report-significant-difference (x y)
+  (if (not (= (- x y) 1))
+      (setf p1-answer (1- x)))
+  y)
+
+(defun solve-5-2 (data)
+  (reduce #'report-significant-difference (sort (boarding-ids data) #'>)))
+
+(defun solve-5 (file)
+  (declare (special p1-answer))  (let ((data (read-5 file))
+	(p1-answer nil))
+    (time (progn (print (solve-5-1 (copy-seq data)))
+		 (solve-5-2 data)
+		 (print p1-answer)))))