I am having a problem when I am trying to copy a formula in excel. I need it to skip rows when copying.

A B
20 10 (This is =A1/2)
40 10 (This is =A1/2)
10 20 (I need this to be =A2/2)

So I want to divide A1/2 in cell B1 and cell B2. Then in cell B3, it should be =A2/2 and cell B4 should be =A2/2. When I drag it down or copy and paste the formula it makes B3 =A3/2. I have been trying to figure it out but I can't get it to work without me changing every cell. Any suggestions would be great.