How to add the invoices amount in a expense report

Hi

I need to add all the invoices amount in an expense report as they are entered. (line by line)

in order to have the total expense amount. (and show it inmediatly to the user)
And once the user send the form record this amount as the expense total.

I can record this amount but I can't show the sum in the form

example
First Invoice $1000 -> total expense $1000
Second Invoice $500 -> total expense $ 1500

Do I explain my self clearly?

Regards