how to counts for every location that select in the dropdownlist that link to gridview?

Good day~
i have a gridview which has category like laptop, keyboard and mouse.
for example, when i choosed Location A in the dropdownlist. meanwhile the gridview will only show out every IT assets(which contain keyboard, mouse and laptop in different model) info in the Location A.
then at the bottom there, a label will show out the total keyboard or mouse or even laptop that have in the gridview.
but how to make it counts for every location that select in the dropdownlist?

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