How to sum POST variables?

Hi
I am trying to do a sum on an update in SC, I have a set value and I want it to take the post inputs, and use one value added to the sum of the other two

{{$_POST.totalcost + ($_POST.unit_cost * $_POST.units)}}

but of course the + symbol doesn’t work in this context, is there a way to do this? I have tried a few things now without any luck.

Thanks
Darren