Can't add up colums to give a cumulative answer with a formula, tried everything I think?

I have 3 rows: new customer ; returning customer and purchases: every month the new customer is a different number, however I want the returning customer to add up for 36 months.

1st month: new customer+returning customer - which is 0
2nd month: new customer+returning customer of month 1
3rd month: new customer +r customer of month 1+2
4th month: new customer+ rcustomer of month 1+2+3
.....
36th month: new customer + rcusotmer of all months added up


here is a link how it looks like : http://s27.postimg.org/q4mt1vahf/Untitled.png

is there a formula by which i dont have to add up all separately? thanks!!!

Try this
=D9+SUM($E$9:E9)
in E9 and apply it for the rest of your column by flash fill or copy/paste
What this does is always add the new customer value from the row and then sum up all the values in the returning customer column from cell E9 (this is locked with the dollar sign $E$9) to whichever row you are on. So the above formula will look like this in E27: =D27+SUM($E$9:E27)

here is the attachment where you can see the excel file

Thank you so much!!!!!!!!!