diff --git a/rename_episode.sh b/rename_episode.sh
index 84c782c..e2a6af1 100644
--- a/rename_episode.sh
+++ b/rename_episode.sh
@@ -22,7 +22,7 @@ for file in "$src"/*; do
   fi
   e=`echo "$file" | grep -o -P "$escheme"`
   if [[ ! -z $eo ]];then
-    e=$(e+eo)
+    e=$((e+eo))
   fi
   ext="${file##*.}"
   echo "ln '$file' '$dest/$name/$name.S$s.E$e.$ext'"