Need formula for cumulative interest with additional monthly princ
 

I am trying to determine the amount of interest paid over a period of time on
a loan. I know how to use the cumipmt formula but I would like to be able to
add in extra monthly payments. For example, say my mortgage payment is $500
per month and I have committed myself to adding an extra $100 each month to
pay it off faster. How would I determine the total interest paid from the
start of the loan to a certain period.