Equation to sum values in columns - differing lengths

[SALBOL1]

Hello -

I'm trying to develope an equation that will allow for the sum of products sold under a promotion, but the lengths of that promotion will vary. If I lock down A1 as the beginng month, but it could go out to F,G,H -- what is the eq I would use to let that varied ending (column) time happen?

Thanks,
Sean

 

[Peter_SSs]

Not sure where your formula will be placed but try this (not in row 1)
=SUM(1:1)