Sum all numbers in a row defined by changing column numbers

Hi,
I am looking for a simple formula that adds us the number of changing columns defined in Cell A1
For example, A1 may be "5" so I need a formula that adds all numbers between columns A2:E2 (5 columns) and
if A1 changes to "6", then I need it to add all numbers between A2:F2 (6 columns) and likewise
if A1 changes to "4", then I need it to add all numbers between A2:D2 (4 columns) and so on...
Can't find anything useful in Google so I though I ask the experts.

You can use this formula (maybe in B1):

=SUM(A2:INDEX(2:2,$A$1))

Hope this helps.

Pete

Thanks Pete. I tried this and it works as I want it but...
in reality, my sum should start from O9 up to BN9, only adding the number of columns defined in $K$1.
So I've placed the new formula in G9 (where required) and changed the formula to:
=SUM(O9:INDEX(9:9,$K$1))
but it returns me a big fat zero - where am I going wrong?

It's always a good idea to describe your actual problem and data layout, rather than simplify it - I was able to use 2:2 (or 9:9 in your case) as the data range to be summed started in column A. You will need to change the formula to this:

=SUM(O9:INDEX(O9:BN9,$K$1))

to relate to your real layout, where K1 contains the number of columns to be SUMmed (i.e. if K1 is 1, this means to SUM(O9:O9) )

Hope this helps.

Pete

Thanks Pete - that worked a treat. Simplifying in this case was not my best option, agreed.

Thankyou friend to make me already join here