calculation with a normal field and a Dropdown (LOCKED)

Hi,

I want to create a Field that give me the amount of product needed according to the length of the site and of the box (selectable in a list)

I've try that :

var a = Number(this.getField("SiteSize").value);

var b = Number(this.getField("DropdownBoxSize").value);

if (b==0) event.value = "";

else event.value = Math.floor(a/b);

but it's not working.

Any idea ?