data var;
input id year var1;
cards;
1 1998 2
1 1999 2
1 2000 3
2 1997 2
2 1998 .
2 1999 4
;run;
data want;
retain var2;
set var;
by id;
if first.id then do ;
if not missing(var1) then var2=var1;
else var2=1;
end;
else if not missing(var1) then var2=var1*var2;
run;