Section have a fixed position when it hits the top of the screen

How to do this it's very simple here example see below .

Jquery Code

var oritop = -100;
$(window).scroll(function() {
var scrollt = $(this).scrollTop();
var elm = $(".scrollTopWindow");
if(oritop < 0) {
oritop= elm.offset().top;
}
if(scrollt >= oritop) {
elm.css({"position": "fixed", "top": 0, "left": 0});
}
else {
elm.css("position", "static");
}
});

Css here code as Sample

body{height:2000px}
.scrollTopWindow {
background-color: red;
width: 150px;
height: 100px;
color: white;
padding:20px;
}

HTML Code here

Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
Some Content here
<div class = "scrollTopWindow"> Scroll top window and fix me ! </div>

Demo Jsfiddle