diff --git a/chapter3/spark.py b/chapter3/spark.py
index 660953f..3b78bcc 100644
--- a/chapter3/spark.py
+++ b/chapter3/spark.py
@@ -46,7 +46,7 @@ def get_last_month(num):
     if month<=0:
         month = 12 - (num-1)
         year -= 1
-    return "%d-%d" % (year, month)
+    return "%d-%02d" % (year, month)
 
 def collect_last_month_sells(spark):
     data = {}