Add sequential number to wordpress menu

Hi.

I need to extend the wp_nav_menu function just a little a bit.

I have a menu structure like this:

<li data-slide="<strong>1</strong>">Home</li>
<li data-slide="<strong>2</strong>">About us</li>
<li data-slide="<strong>3</strong>">Portfolio</li>
               .
               .
               .
               .
and so on

What I need, is to add to each list the data-slide attribute, and a sequential number, as seen above.

Thank you.