Count Week numbers given start and finish week number

Dear forum,
Attached a file with week numbers in column BM and BM. My aim was to count the week numbers from week start(BM) to week end (BN). I have inserted following VBA and it works.

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The problem is that in my file I have more than 1000 lines and when applying the code , as mention in the code it clears all the 1000 lines and re count again though i have lines without any changes (Start, end week numbers) and it takes lot of time. Please help me to modify the code so that the VBA apply only to lines where there have been changes in week start and week end numbers so that the VBA catter only for changes or addition of new lines.
Thanking you for your help.

The changes in weeknumbers, are you do this manuel or delivered by formula ?

Kind regards
Leo

Hi Leo,
It is by formula.

Hi bakerman2,
Thanks for the proposal, I have tested it, display result very quick. The problem is that it remove the formula in BM and BM ( Calculate week number with respect to date in BK and BL. These date also are the result a logical formula inserted meaning that these dates can change if one of the parameters changes. Can we do same action without removing the formula in BM & BN. Many thanks for your help.

Hi bakerman2,
It works perfectly and is very quick. At the end of each click on the button it gives a message box named Microsoft Excel with a figure in it (not same every time , i,e 0.00645 etc) What does it mean? can we remove it.
Thank
Vassen

Hi bakerman2
Super, perfect match of what I required, many thanks, I have not been able to add reputation(It does not allow me to do so) Thanks again
Cheers
Vassen

Hi Bakerman,
In the file i was working there were some empty cells in betweem filled ones in column O and P, and when delted, VBA was ok , it works perfectly. Thanks you for your help
Regards
Vassen