Loop with a lag function

[eemrun]

I need to calculate rows with a lag variable but struggling to do that. Essentially it should follow this function

For first row, D = A+B

For subsequent rows, D = lag(D)+A

Any guidance?

Date	A	B	C	D
202303	254509298.4	5644500000		5899009298
202304	125121716.6		5899009298	6024131015
202305	224951659.8		6024131015	6249082675
202306	189510381.8		6249082675	6438593057
202307	225507645.3		6438593057	6664100702
202308	332878952.2		6664100702	6996979654
202309	69832226.1		6996979654	7066811880
202310	155360032.1		7066811880	7222171912
202311	212155525.5		7222171912	7434327438
202312	104031780		7434327438	7538359218
202401	67473757.38		7538359218	7605832975
202402	120658625.5		7605832975	7726491601

[Tom · Accepted solution]

No LAG() needed.  Just make sure D is a NEW variable and RETAIN its value.

data want ;
  set have;
  retain d;
  if _n_=1 then d=a+b;
  else d=d+a ;
run;

[eemrun]

Oh brilliant! So simple. Thanks so much!