How to find the highest z-index using jQuery

I have a number of div elements with different z-index. And I want to find the highest z-index among these divs - how can I achieve it?

CSS:

#layer-1 { z-index: 1 }
#layer-2 { z-index: 2 }
#layer-3 { z-index: 3 }
#layer-4 { z-index: 4 }

HTML:

<div id="layer-1">layer-1</div>
<div id="layer-2">layer-2</div>
<div id="layer-3">layer-3</div>
<div id="layer-4">layer-4</div>

I don't think this line can find the highest z-index though.

var index_highest = parseInt($("div").css("zIndex"));
// returns 10000