Counting every other row - DSUM? SUMPRODUCT?

I have a report where I need to create a calculation to sum every other cell in a column.  I was successful in getting results using the following formula =SUMPRODUCT((MOD(ROW(\$AM\$11:\$AM\$500),2)=0)*(\$AM\$11:\$AM\$500)) .  I also tried using =MOD(ROW(AM9),AS2)=0.

Using the first formula I was able to match the sum that I have by selecting the cells manually for all of the even numbered row in the range.  The second formula resulted in a 0 sum.   My problem is that I have numbers in the odd cells that I need to sum as well. Is it possible to use either of the formulas from above to get the results without having to manually select the cells?
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Commented:
When entering this formula...

Press : CTRL+SHIFT+ENTER.  This will enter the formula below as an array formula.

=SUM(IF(MOD(ROW(AM11:AM500),2)=0,AM11:AM500,0))

see: http://support.microsoft.com/kb/136738
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Commented:
This version sums every other row starting with the first row in the range (in your case that will be the odd rows)

=SUMPRODUCT((MOD(ROW(\$AM\$11:\$AM\$500)-ROW(\$AM11),2)=0)*1,\$AM\$11:\$AM\$500)

change the 0 to a 1 and it sums all the others

=SUMPRODUCT((MOD(ROW(\$AM\$11:\$AM\$500)-ROW(\$AM11),2)=1)*1,\$AM\$11:\$AM\$500)

Note: the extra ROW function makes the formula more robust in that it will still give the correct results even if you add or delete rows in the worksheet

regards, barry
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