$(document).ready(function () {

    var positionMain = function () {
        var mainHeight = $('#main').height();
        var windowHeight = $(window).height();
        if (windowHeight > mainHeight+40) {
            $('#main').css("padding-top", (windowHeight - mainHeight) / 2);
            $('#main').css("padding-bottom", (windowHeight - mainHeight) / 2);
        } else {
            $('#main').css("padding-top", "20px");
            $('#main').css("padding-bottom", "20px");
        }

    }
    positionMain();
    $(window).resize(positionMain);


});