Store Sales Average calculation help request

Dear Experts.

I have a Store slicer with the single selection option in it. My report displays the sales information of that Store week wise. I was asked to get the average sales for each week.

Average Sales = Total Sales(for all the stores) / Total Sales of the store(selected on the slicer) for that period.

Please can someone help me with your valualble input to achieve this.

Thanks
Mahad