function check_size() {
	if ($(window).height()<600) {
		$("#main").css("top","0px");
		$("#main").css("margin-top","0");
	} else {
		$("#main").css("top","50%");
		$("#main").css("margin-top","-300px");
	}
}
$(document).ready(function(){
	check_size();
});

$(window).resize(function(){
	check_size();
});