1 files changed, 3 insertions, 1 deletions
diff --git a/09/main.py b/09/main.py
index 7fb235f..f53b218 100755
--- a/09/main.py
+++ b/09/main.py
@@ -17,8 +17,10 @@ for line in sys.stdin:
     derivatives.append(derivative(derivatives[-1]))
 
   prediction = 0
+  sign = 1
   for d in derivatives:
-    prediction += d[-1]
+    prediction += d[0] * sign
+    sign *= -1
 
   output += prediction