Regarding total year sales

I am getting actual sales for month customer wise from s001-ummenge field.

i want to accumlate the every month sales for year for customer wise.

my year wise sales is not directly coming from the system.

it is coming with following way...

suppose i am getting january 100

february 200

march 300

april 450

like that upto december i will get differnt quantity.

final i want to display total from january to decemeber.

that means 100 + 200 + 300 + 450 + ..... = 950 ..

i want to display my total target per year is 950.

how to write code for above logic.

pls suggest me..