Adding class to alternate divs?

Adding class to alternate divs?

Hi,

Is there any way to add a class to alternate DIVs (or any other tag, eg. dd)?

For eg.

<div id="root">
<div>one</div>
<div>two</div>
<div>three</div>
<div>four</div>
</div>

I'd like to add a certain class to two and four DIVs, is that possible?

Thanks in advance.