Summation of Key Figure

Hi,

I have a following requirement :-

I need to achieve this using BEx Query Designer.

Total Plant Volume(TPV) is calculated as such

KF1 + KF2 + KF3.

If this has value, then the result is this value.

ELSE

KF4 + KF5 + KF6

If this has value, then the result would be this value

Only if neither of the above 2 has results,

Then it should be KF 7

I don`t think we could just sum (KF1 +... KF6) because the ORDER of the summation is important.