Sum formula to exclude monthly budget figure when actual figure is entered

Hi - I am an excel novice - and need a formula which will add up the monthly figures in a budget.
That is - at the beginning of the financial year, the total will be the 12 months of budgeted expenditure. However as the year progresses, and an 'actual' expenditure figure is added in month 1, this needs to be included instead of the budgeted figure.

I have created a very long and messy formula which works, but I am keen to learn, and know there must be a tidier and simpler way to do it. I have played around with SUMIFS with no joy.

This is my long formula. I can imagine your laughter!! A2, C2, E2 etc are actual expenditure figures, B2, D2, F2 figures are budgeted expenditure.

=IF(A2>0,-B2)+IF(C2>0,-D2)+IF(E2>0,-F2)+IF(G2>0,-H2)+IF(I2>0,-J2)+IF(K2>0,-L2)+IF(M2>0,-N2)+IF(O2>0,-P2)+IF(Q2>0,-R2)+IF(S2>0,-T2)+IF(U2>0,-V2)+IF(W2>0,-X2)+SUM(A2:X2)

Thanks so much for any ideas.

Sure. Sample attached. Thanks again for any help - I appreciate it.Budget versus actual.xlsx