How to Calculate tha difference from selected cells?

Hi,

I have a concept according to that, I want to calculate difference.My concept is, I have made 3 columns, in the 3rd column it will sum-up the "previous and current value". Towards my right hand side, I have another column, which will calculate my savings, in the same way as I used for calculating the 3rd column.Also I want "Average" to be calculated in separate cell.(Basically my concept is to know the difference between my "savings" and my "expenses" in a particular month)Can anybody help me on this concept.


Thanks,
Regards,
Phaneendra.