diff --git a/day12/sol.py b/day12/sol.py
index e509adb..31074bf 100644
--- a/day12/sol.py
+++ b/day12/sol.py
@@ -28,6 +28,7 @@ def solve(file):
 
     done = set() # set of points which have been counted as part of a shape
     total = 0
+    total2 = 0
     for p in points():
         if p in done: continue
         shape = flood(p)
@@ -38,8 +39,33 @@ def solve(file):
             perim += 4-n
         #print(get(p), area, perim)
         total += area * perim
+        total2 += area * discount(shape)
 
     print(total)
+    print(total2)
+
+def discount(shape):
+    def sides(p):
+        x,y = p
+        s = 0
+        if (x+1,y) in shape: s += 1
+        if (x-1,y) in shape: s += 2
+        if (x,y+1) in shape: s += 4
+        if (x,y-1) in shape: s += 8
+        return s
+
+    # only count a side as free if the neighbor to the north or west
+    # does not have the same side free
+    perim = 0
+    for p in shape:
+        x,y = p
+        free = sides(p) ^ 15
+        if (x-1,y) in shape:
+            free &= sides((x-1,y))
+        if (x,y-1) in shape:
+            free &= sides((x,y-1))
+        perim += free.bit_count()
+    return perim
 
 solve(open("sample4.in"))
 solve(open("input"))