Adding values in textbox, checkbox and dropdown menu

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Javascript

Java

HTML

I have this program that will add the value in a textbox, checkbox and dropdown menu. The way it run is, the textboxes will be added, then if the customer checks the checkbox, it will be added to the payment if not checked, no additional payment. then the drop down menu, whatever the customer will choose, it will be added to the payment.
How will I write my code to get the total of all the values?

These are my codes so far.

function subtotal(){
    var a = document.getElementById("sff").value;
    var b = document.getElementById("osf").value

    var stotal = a+b;
    document.getElementById("st").innerHTML = "$" + stotal;

}</pre>

<pre lang="HTML"><input type="text" id="osf" placeholder="$0.00" style="float:left;" onchange="subtotal()"/>
<input type="text" id="sff" placeholder="$0.00" style="float:left;" onchange="subtotal()"/>
<select style="margin: 3px 0 0 55px; width:48%; position: absolute; float: left; ">
                    &lt;option value="">(Select Number Of Years Of Agent Service To Purchase)&lt;/option>
                    &lt;option value="a1">1 Year @ $99.00/yr (Service to December 2015)&lt;/option>
                    &lt;option value="a2">SAVE! 2 Years (@ $89.10/yr) for only $178.20 (Service to December 2016) - Save $19.80&lt;/option>
                    &lt;option value="a3">SAVE! 3 Years (@ $80.52/yr) for only $241.56 (Service to December 2017) - Save $55.44&lt;/option>
                    &lt;option value="a4">SAVE! 4 Years (@ $73.06/yr) for only $292.24 (Service to December 2018) - Save $103.75&lt;/option>
                    &lt;option value="a4">SAVE! 5 Years (@ $66.56/yr) for only $332.80 (Service to December 2019) - Save $162.20&lt;/option>
                &lt;/select>
<input type="checkbox" id="cbx2"/>($35.00) - Prepare IRS Form SS-4 (Federal EIN Number)</pre>