Working with math in javascript

Working with math in javascript

I get the value 17.000, 17 thousands.
var pay = document.getElementbyId("name").InnerHTML;
alert(pay);
it shows "17.000". But if I do like this:
var f = (pay * 24);
alert(f);
it shows 408, not 408.000. It makes 17 * 24 not 17.000 * 24. How to work
around this?