I am having trouble wrapping my brain around this one.

I have a listing of columns for months (in reverse order). In each row
is that months budget numbers. What I want to do is have a column that
sums the year to date based on a value that is typed into a cell.
Colunn U = Jan
column T = Feb
Column S = March
etc

So if I type a 1, I want the sum of row x columns U:U
If I type a 3, I want the sum of row x columns S:U
etc

Thanks in advance for any help.
Guy