Declare @STR varchar(5)

select @STR = ''

select @STR = @STR + no from mystring

step 1: '' + '1'

step 2: '1' + '2'

step 3: '12' + '3'

step 4: '123' + '4'

step 5: '1234' + '5'

so if i apply ((N^2) + N)/2 formula

N = 5

...

5^2 = 5*5 = 25

25+5 = 30/2 = 15. Right?

O(n) operation where n = Len(A$)+Len(B$).