Find top level li with jQuery

Find top level li with jQuery

Let's say i have a menu like this...
<ul class="main_menu">
<li class="item-1">
First top level item
<ul>
<li class="item-2">Item 2</li>
<li class="item-3">
Item 3
<ul>
<li class="item-4">Item 4</li>
<li class="item-5">Item 5</li>
</ul>
</li>
<li class="item-6">Item 6</li>
</ul>
</li>
<li class="item-7">
Second top level item
<ul>
<li class="item-8">Item 8</li>
<li class="item-9">Item 9</li>
<li class="item-10">Item 10</li>
<li class="item-11">Item 11</li>
</ul>
</li>
</ul>
...and in this case the sub menus can be at variable depth, how can i get
the top level item if i only know an sub item? For example, i know the
class item-5, then i want to get the class "item-1", or if i know the
"item-11", i want "item-7".
That is, wherever i am, i want the "Top level item X".