It looks like the extra read is your best option - https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.OperatorsAndFunctions.html#Expressions.OperatorsAndFunctions.Syntax says that the grammar only supports comparator operators, not arithmetic operators.

I don't know if there's an underlying technical reason for this though - hopefully not, and hopefully one day DynamoDB supports this.