Why don't arithmetic operations work in case in ms sql?

Question

SQL Server

I can't understand why datepart(ww, part_date) - 1 is not being subtracted in the following construction

select 
case  
when datepart(weekday, part_date) = 1 then datepart(ww, part_date) - 1 else datepart(ww, part_date) as part_week
end

The condition itself works, if instead of datepart(ww, part_date) - 1, just write 1, then everything is replaced. Tried cast and convert to int , still one is not subtracted from datepart(ww, part_date) value

Answer 1

Data types in the studio!

Answer 2

-1 + datepart(ww, part_date) - this is how it works