seq_nbr     value       summation
----------- ----------- -----------
1           5           5    (5)
3           8           13   (5 + 8)
4           4           17   (5 + 8 + 4)
8           2           19   (5 + 8 + 4 + 2)
11          6           25   (5 + 8 + 4 + 2 + 6)

declare @t table
(seq_nbr integer not null primary key,
 value integer not null);

insert into @t (seq_nbr, value)
values (1,5), (3,8), (4,4), (8,2), (11,6);

with noGaps as
(
      select row_number() over(order by seq_nbr asc) as seq_nbr,
             value
        from @t
),
cteSource (seq_nbr, value, summation) as
(
      select seq_nbr, value,
             value as summation
        from noGaps
       where seq_nbr = 1
       
       union all       

       select g.seq_nbr, g.value,
              c.summation + g.value as summation
         from cteSource as c
              inner join noGaps g
                 on c.seq_nbr + 1 = g.seq_nbr
)
select * from cteSource
option (maxrecursion 0);