diff --git a/04b.lisp b/04b.lisp
new file mode 100644
index 0000000..8f39b46
--- /dev/null
+++ b/04b.lisp
@@ -0,0 +1,70 @@
+(require :uiop)
+(load (sb-ext:posix-getenv "ASDF"))
+(asdf:load-system 'str)
+
+(defun read-diagram (lines)
+  (let* (
+         (dim (length lines))
+         (diagram (make-array (list dim dim))))
+    (loop for line in lines
+          for y from 0
+          do (loop for char across line
+                   for x from 0
+                   do (if (char-equal char #\@)
+                          (setf (aref diagram y x) 1))))
+    diagram))
+(read-diagram (uiop:read-file-lines "04example.txt"))
+
+(defun make-kernel (diagram -x -y)
+  (let ( (kernel (make-array '(3 3))))
+    (loop for y from -1 to 1
+          do (loop for x from -1 to 1
+                   do (let ((-x (+ -x x))
+                            (-y (+ -y y)))
+                        (if (array-in-bounds-p diagram -y -x)
+                          (setf (aref kernel (+ y 1) (+ x 1)) (aref diagram -y -x))
+                          (setf (aref kernel (+ y 1) (+ x 1)) 0)))))
+    kernel))
+(make-kernel (read-diagram (uiop:read-file-lines "04example.txt")) 0 2)
+
+(defun sum-kernel (kernel)
+  (and
+    (= (aref kernel 1 1) 1)
+    (<
+      (-
+        (loop for y from 0 to 2
+          sum (loop for x from 0 to 2
+                    sum (aref kernel y x)))
+        1)
+      4)
+    ))
+(sum-kernel (make-kernel (read-diagram (uiop:read-file-lines "04example.txt")) 2 0))
+
+(defun sum-diagram (lines)
+  (let ((dim (- (length lines) 1))
+        (diagram (read-diagram lines))
+        (subtotal 0)
+        (total 0))
+    (loop
+      do
+      (progn
+        (setq subtotal (loop for y from 0 to dim
+             sum (loop for x from 0 to dim
+                       sum (if (sum-kernel (make-kernel diagram x y))
+                             (progn
+                               (setf (aref diagram y x) 0)
+                               1)
+                             0))))
+        (setq total (+ total subtotal))
+        )
+      until (= subtotal 0)
+      ;; do (progn
+      ;;      (print diagram)
+      ;;      (print subtotal)
+      ;;      (print total)
+      ;;      (sleep 1))
+      )
+    total
+    ))
+(sum-diagram (uiop:read-file-lines "04example.txt"))
+(sum-diagram (uiop:read-file-lines "04data.txt"))