I want to get order of li that clicked, but output is -1 . but i want to click any img if it is first of "ul" it alerts 1, if it is second of "ul" it alerts 2?

<ul class="w1m"> <li>boston</li> <li>dallas</li> <ul> <li><img src='img/minussub.png' /></li> <li><img src='img/minussub.png' /></li> </ul> </ul>

$("li").on("dblclick","img[src='img/minussub.png']", function() { alert($(this).closest('ul.w1m').children('ul').index('ul.w1m > ul > li')); });​