declare @t table(k int primary key, v int)
insert into @t
values (1, -1), (2, 1), (3, 2), (4, 2), (5, 7), (6, 8), (7, 4), (8, 1), (9, 3), (10, 0), (11, 8), (12, 9), (13, 10), (14, 11);

declare @start_i int = 0;
declare @end_i int = 9;

select case when d.first_v > @start_i then @start_i else d.bounded_v + 1 end as available_v
  from (select min(c.this_v) as first_v, max(c.this_v) as last_v,
               min(case when c.next_v - c.this_v > 1 then c.this_v
                        when c.next_v is null        then c.this_v end) as bounded_v
          from (select max(case when e.x  = (b.r % 2) then b.r end) as this_r,
                       max(case when e.x  = (b.r % 2) then b.v end) as this_v,
                       max(case when e.x != (b.r % 2) then b.v end) as next_v
                  from (select a.v, a.k, row_number() over(order by a.v) as r
                          from @t as a
                         where a.v between @start_i and @end_i) as b
                         cross join (select 0 as x union all select 1 as x) as e
                  group by b.r + case when e.x = (b.r % 2) then 1 else 0 end) as c) as d
 where not(d.first_v = @start_i and d.bounded_v = @end_i)

declare @boundz table(id tinyint primary key, start_i int, end_i int, check(id = 0 and start_i < end_i))
insert into @boundz 
values (0, 0, 9);