[jQuery] How do I subtract 64 pixels from the value of a div's height?

[Jeremy Coulson]

Here is my code:

$(document).ready(function () {
        var contentDivHeight = $('#contentDiv').css('height');
        // pseudo-code: var calculatedHeight = ????
        $("#clickMe").click(function(){
                $("#leftNavBox-contentPages").animate({"height": 
calculatedHeight},
"fast");
        });
});

I need the value of calculatedHeight to be 64 pixels less than
contentDiv.  So, if contentDiv is 600px, calculatedHeight needs to be
536px.

Jeremy