I've got a row of numbers A2:K2. I want a formula that will sum from A2 to
the number of cells I enter in A1.

So if I enter "3" in cell A1, the formula will sum from A2 to C2.
and if I enter "5" in cell A1, it will sum from A2 to E2.

I've been fiddling around in R1C1 but not getting it to work...