How to fix NaN

I trying to do a simple calculation with the selection the user has made; however, I am getting a NaN
when the calculation is performed. What can I do to fix it. I only want the see the sum appears in the message box

Thank you

drop down menu:


<SELECT name="Service" onchange="ServiceItemSelected()"size="1">
<option value = 0 selected> Choose A Service Level.
<option value = 1 >Internet/Digi</option>
<option value = 2 >Internet/Vid</option>
<option value = 3 >Internet Only</option>
<option value = 4 >Digital</option>
<option value = 5 >B2</option>
<option value = 6 >B1</option>

</SELECT>
Now the JavaScript

<Script language='javascript'>
function ServiceItemSelected(){
var serviceSelected;
serviceSelected = serviceSelected + 0.4;

var itemSelected;
var selectObject;
selectObject = document.getElementById("Service")
itemSelected = selectObject.options[selectObject.selectedIndex].Text
if (itemSelected == "Internet/Dig")
{
serviceSelected = serviceSelected *= 4;
}
if (itemSelected == "Internet/Vid")
{
serviceSelected = serviceSelected *=3;
}
if (itemSelected == "Internet Only")
{
serviceSelected = serviceSelected *= 2;
}
if (itemSelected == "Digital")
{
serviceSelected = serviceSelected *=2;
}
if (itemSelected == "B2")
{
serviceSelected = serviceSelected *= 1;
}
if (itemSelected == "B1")
{
serviceSelected = serviceSelected *= 0;
}
var totalSelected;
totalSelected = parseFloat(serviceSelected + itemSelected);

alert(totalSelected);

}

</script>

your code is all messed up and I can't see any sum calculation... to calculate sum have such code:
strSum=0;
strSum += 8;
strSum += 3;
...
alert(strSum);

this will put sum of 8 and 3 in the variable strSum for example.