Sum statement not overcoming missing values

[ManitobaMoose]

This should be easy for an experienced programmer.

I am learning the sum statement with missing values. According to what I understand in my book, the Sales + sumsales statement should overcome missing values. Yet, I am only getting missing values for sumsales. Any ideas what I may be doing wrong?

Thanks!

-----------------------------------------------

Data monthsales ;
    input month sales @@ ;
        Sales + Sumsales ;
        output ;
datalines ;
1 4000 2 5000 3 . 4 5500 5 5000 6 6000 7 6500 8 4500 9 5100 19 5700 11 6500 12 7500
;
run ;

proc print Data=monthsales ;
    var Sales Sumsales ;
run ;

[Patrick]

@ManitobaMoose

Instead of 

Sales + Sumsales ;

write

Sumsales + Sales;

[ManitobaMoose]

Excellent advise. That works perfectly. Thanks!

[ManitobaMoose]

Advice.