U-SQL subquery question

Guys I'm having a hard time trying to figure out how to do the next subquery

```sql
declare @ventas as table(
semana int,
existencia int
);

declare @semanas as table(
semana int
);

insert into @ventas
values (1, 10),(3, 9),(5, 8);

insert into @semanas
values (1), (2), (3), (4), (5);

select s.semana,
isnull(v.existencia,
-- **** this part
(select top 1 existencia from @ventas where semana < s.semana order by semana desc) ) as existencia
from @semanas s
left outer join @ventas v on v.semana = s.semana