horizontal summing in Excel

[jimsmyth]

I would like to have a running total in Excel where I change weekly numbers in a column or two yet my last column keeps the year to date numbers.  i.e.  If A1 is 6 one week, B1 is 2 that week, C1 should be 8.  The next week, A1 is 3, B1 is 4, C1 should be 15.  Does anyone know a formula for this?  Thanks.

[#2]

@jimsmyth 

If in C1 8, in C2 15, etc, when in C1

=SUM($A$1:$B1)

and drag it down.

If sum for entire range is always in C1, when like

=SUM(A1:INDEX(B:B,COUNTA(B:B)))