SUM values after substraction

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MySQL

Hello,

I have a question regarding my SQL statement. I want to sum the value of differenz.
How di I do this?

SELECT a.spielername AS spielername, count(a.koord) AS anzahl, b.name, b.planeten AS planeten, b.planeten - count(a.koord) AS differenz
                     FROM vs_universum a
                     INNER JOIN vs_highscore b
                     ON a.spielername = b.name
                     WHERE b.datum = :datum
                     GROUP BY b.name
                     HAVING count(a.koord) <> b.planeten

Thank you very much

With kind regards

Tobi

What I have tried:

I tried to sum differenz but this throws an error.