diff --git a/ac_encode.py b/ac_encode.py
index 7592d17..57aacd2 100644
--- a/ac_encode.py
+++ b/ac_encode.py
@@ -82,8 +82,10 @@ def encode(string, c0=BETA0, c1=BETA1, adaptive=True):
         # if the gap a-b is getting small, rescale it
         while a > QUARTER and b < THREEQU:
             charstack[0] += 1
-            a += a - HALF
-            b += b - HALF
+            a *= 2
+            b *= 2
+            a -= HALF
+            b -= HALF
 
         assert a <= HALF
         assert b >= HALF