Sum with a text value from other cell

Hello,

I have a simple formula, and this is

=sum('6m'!B2:B85)

6m is my sheet. But I want to change with a value from other cell.

Something like this

=sum(' &A1& '!B2:B85)

Inside A1 cell has 6m ( the name of the sheet)
I want to text inside A1 be evaluate and put as part of the formula code.


I think it is simple, but I am having trouble to search.
I tried text inside formula excel. But I am not getting anywhere near. So I decide to ask for help.

Thanks