Here's a simple example of something I'm trying to figure out.
A B
1 5
1 7
2 4
2 9

Can I calculate (5*4)+(7*9)=83 using a sumproduct?

I tried this but it doesn work for what I think is an obvious reason.
SumProduct(((B1:B4)*(A1:A4=1)),((B1:B4)*(A1:A4=2)) ). I believe this isn't
working because there is never a case where A1:A4=1 AND A1:A4=2. Therefore I
get the answer = 0.

I'm also having a hard time trying to figure out how I would do this in a
loop of some kind as an alternative. Can I define B1:B2 as a range and B3:B4
as a range and pass them to a function that computes B1*B3 + B2*B4? How do I
define those ranges in code?

Thanks for your help.