Summing every 2 rows in blocks

Hi all,

maybe a simple problem, but can't seem to get it.
I am trying to convert half hourly data into hourly, so I need to sum two 30min blocks to get an hourly block.
But when I drag the formula down it counts the previous value as well.
E.g. in the tab Hourly - cell B2 - i have input the logic to sum B2 and B3 from tab half hourly, but then when i drag that formula down it will add B3 and B4 - I would like it to add B4 and B5, and so on, i want to calculate every 2 rows without incorporating the previous row.
Hope that makes sense.

I have attached my spread sheet to explain.

thanks for your help.

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