Javascript for handling two pulldowns & total price

Need a javascript for a form that will handle two pulldowns with 4 values in each pulldown (e.g. 1, 2, 3, 4) and will automatically add to the existing total price field in the form. Example: existing total price value in the form will exist in something like: <input type="hidden" id="Total" value="100"> . Everytime the user selects from one of the two pulldowns, this Total will be updated accordingly.