=MOD(B1,2) returns 1 for odd rows and 0 for even row. Do this calculation in column C.

Then use =SUMIF(C1:C37,0,B1:B37) to sum even rows, or =SUMIF(C1:C37,1,B1:B37) for odd rows.

This provides a simpler formula that is easy to follow and should be easier to troubleshoot.