Convert Activity into KF by creating a Formula Variable(replacement path)
Create a New Selection(Activity KF) and drag above KF and restrict with Activity = PTWY. Hide this whole KF.
Create a new Formula(Activity KF1) with above New Selection inside + 0
Create one more new Formula C = A * B
Create an If ..else like below:--
( Activity KF1 < > 0 ) * C + ( Activity KF1 = = 0 ) * A