Math and Custom Fields

I am using the following code to create a variable that references a custom ID:

<? $prodid = get_post_meta($post->ID, 'product-id', true) ?>

Later on, I want to use the same custom field, but multiply it by 2. Is this possible with PHP? I am thinking something ike this:

<?php $prodid*2 ?>

I know this is far from correct, however, I know it is something simple; I just don’t know enough PHP to figure it out!

Thanks so much!