This should answer it :

DECLARE @a int, @b-2 int, @c int, @d int

Select @a = 1, @b-2 = @a + 1, @c = @b-2 + 1, @d = @c + @b-2 + @a

Select @a as a, @b-2 as b, @c as c, @d as d

a           b           c           d          

----------- ----------- ----------- -----------

1           2           3           6

(1 ligne(s) affectée(s))