=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.
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