Sum of the product of 2 columns

Hi,

I have a 2-column table. Column "quantity" and "per Unit". For each row I want to multiply "quantity" by "per Unit", then sum all of that, then update at the footer. I could not find any example, struggle for few days already. I suspect that I could use columns().data() but don't know how to apply. Basically reduce from 2 dim to 1 dim then to single value.

Appreciate for any help. Thanks.