I want to lookup a moving range in a table.

account week 1 week2 week 3 This week week 5
a 1 2 3 4 5
b 1 3 4 5 6
c 2 3 3 5 5
d 2 2 5 3 2

So I want to use a looukp to say the sales for account b this week and last
week were 9 units. I will only update the this week columm so I want my
formlua to search for "this week" and return the value for the sum of "this
week" and "this week" -1.