Sorry kiddo, there was some syntax error so I modifed and came to my way. I am using python 3.2.3.
There is extra sign at last of the series.
Here is result of your last code.
N: 5
1 - 2 + 3 - 4 + 5 - = 3
>>> ================================ RESTART ================================
>>>
N: 4
1 - 2 + 3 - 4 + = -2
>>>




prankster,
your code don't render 1 at begining and series start from - sign, same problem as we discuss here. Here is the result.
>>> ================================ RESTART ================================
>>>
N: 5
-
2
+
3
-
4
+
5
= 2
>>> ================================ RESTART ================================
>>>
N: 4
-
2
+
3
-
4
= -3