How do I select the following string so it returns 3? SELECT '1+2', Sql Server

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create function [dbo].[delimitedsplit8K] (
      @pstring varchar(8000)
    , @pdelimiter char(1)
  )
returns table with schemabinding as
 return
  with e1(N) as (
    select 1 union all select 1 union all select 1 union all 
    select 1 union all select 1 union all select 1 union all 
    select 1 union all select 1 union all select 1 union all select 1
  )
  , e2(N) as (select 1 from e1 a, e1 b)
  , e4(N) as (select 1 from e2 a, e2 b)
  , ctetally(N) as (
    select top (isnull(datalength(@pstring),0)) 
      row_number() over (order by (select null)) from e4
  )
  , ctestart(N1) as (
    select 1 union all
    select t.N+1 from ctetally t where substring(@pstring,t.N,1) = @pdelimiter
  )
  , ctelen(N1,L1) as (
    select s.N1,
      isnull(nullif(charindex(@pdelimiter,@pstring,s.N1),0)-s.N1,8000)
    from ctestart s
  )
 select itemnumber = row_number() over(order by l.N1)
      , item       = substring(@pstring, l.N1, l.L1)
   from ctelen l
;
go
CREATE TABLE #theSums (SumStrings VARCHAR(25))
INSERT INTO #theSums
    values
        ('1+2'),
        ('1+2-3');
        
select 
    t.SumStrings
  , Summed = sum(convert(int,s.Item))
from #theSums t
cross apply delimitedsplit8K(replace(replace(SumStrings,'+','|+'),'-','|-'),'|') s
group by t.SumStrings

View schema
Execution time: 0,03 sec, rows selected: 2, rows affected: 2, absolute service time: 0,17 sec