Update many while multiplying fields

I need to updatemany while multiplying fields is sucj thing possible?, I would like to do something like this command:
“db.length.updateMany({},{$set:{totalPay:{$multiply:[“$km”,”$price”]}}})” this command oviously not Valid and i dont know why, maybe cause multiply is an aggregation function?

How would u approach such simple task?

What do you get? An error? Wrong result?

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