return index of chaged handle on jquery-ui slider

I have a jquery ui slider :

$( "#slider" ).slider({
values: [ 10, 25,45,176 ],
max:190,
change: function( event, ui ) {  
        var index = $("#slider span").index(ui.handle);
        $( "#index" ).text( "That was handle index #" + index );
    }
});

i want to return the index of changed single handle. but handle object returns all spans ( handles). how can i do that?