I am trying to develop a sales projection worksheet. There are a
number of programs we sell each providing a different number of months
of service. What I want to do is have the user input the number of
sales for each month for each program in a row. I then want to total
the number of subscribers for each month by adding that value to each
month for the duration of their subscription. The number of months
duration is defined in a separate cell. I have been playing around
with this for several days and getting nowhere fast. Any help would be
greatly appreciated.

Sales Jan Feb Mar Apr

6 Month Subscription 1 0 0 0
12 Month Subscription 1 0 0
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# Subscribers 1 2 2 2
.... ....