So given the following:

declare @a int

declare @b-2 varchar(2)

set @a = 10

set @b-2 = ' '

IF ISNUMERIC(@b) = 1

select @b-2 +@a + 2;

ELSE Print 'Unknown'

-- Even though the following is the result

select @b-2 +@a + 2;

The result is 'Unknown' after which the actual result, 12.