Queries :: Query Count Difference Of Two Fields?

i have a access table (AUTH) with following field

Company Auth Held

Tata 12

Dell 11

HP 21

Opera 11

Bangour 10

i used the following query to calculate the Held from a table named pers.

SELECT AUTH.company, AUTH.Auth, Count(AUTH.company) AS Held
FROM pers INNER JOIN AUTH ON pers.company = AUTH.company
GROUP BY AUTH.company,AUTH.Auth

then i got the result as under

Company Auth Held

Tata 12 02

Dell 11 08

HP 21 20

Opera 11 12

Bangour 10 12

now i want the difference between Auth and Held as Sur/Defi

how can be it done.