Adding Up Numeric Stepper

Hi all,
I’m making a t-shirt site and I have several numeric steppers that let the user enter a vaule for how many smalls, mediums ect. that they want.

I want to show the total number of shirts in the order in a textField called:

tfTotal

I really thought that something like this would be a common thing that others would do, but i can’t find it anywhere. below is part of the code:

var aSmall = new Object();
aSmall.change = function() {
	trace(cnsAS.value);
};
cnsAS.addEventListener("change", aSmall);

var aMedium = new Object();
aMedium.change = function() {
	trace(cnsAM.value);
};
cnsAM.addEventListener("change", aMedium);

I would love any help you guys could give me. Thanks!