How to write a formula which accesses a cell value from the just previous sheet

Hello,

I have an excel workbook 2010, in which i have a number of sheets.
In each sheet, I have the results of a particular test run and the analysis.
Now, for the analysis, I need to do a diff of the number of test-cases passed in the current run (Current Sheet) and the number of test-cases passed in the previous run (previous sheet).
And this formula I need for all the sheets.

Thus, how do I have a formula in which I do a diff of a value of the current sheet and the previous sheet.
I case of just two sheets (sheet1 and sheet2), if I have to do a diff on sheet2 then the formula is :
=B4-'sheet1'!B4
But this is when my sheet names are constant, but I need the formula such that for each new sheet, the formula picks the value from the previous sheet only.

Could someone please help me with this?
Also, I do not know how to create macros in excel, so if you provide the code for a new macro, I would be grateful if you could explain how to use/apply it also.

Thanks and regards,
Sunny

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