Excel formula help

Hello! I used this formula successfully on a cell: = If (A2 = "MA1"; sheet1! D1; sheet2! B1)

                 Now, I want to apply the same formula on the other cells of the lower lines so that it gives me this: = If (A2 = "MA2"; sheet1! D2; sheet2! B2)

                But my problem is how to design the same formula so that at each cell, "MA1" automatically changes to "MA2", and then to "MA3" ...