Custom js isn't working

Hi,
Everything is ok but only javascript not working. I don’t know where can be problem, only it could be bad code.
This is my CSS:

html,body{
	margin:0;
	padding: 0;
}
html {
	box-sizing: border-box;
}

*, *:before, *:after{
	box-sizing: inherit;

}

.container:before,
.group:before,
.menu:before,
.container:after,
.group:after,
.menu:after {
	content: " ";
	display: table;

}

.container:after,
.group:after,
.menu:after{
	clear: both;
}

.container,
.group,
.menu{
	*zoom: 1;

}

/* ----------------------> Header <-------------------------- */
.menubar {
	width: 100%;
	height: 87px;
	background: #cccccc;

}

.logo h1 {
	float: left;
	font-size: 50px;
	margin: 0;

}

#menu-header
	{
	margin: 0;
}
#menu-header ul {
	position: relative;
	font-family: Arial;
}

nav ul ul.sub-menu {
	position: absolute;
	margin-top: 20px;
	display: none;
}

nav ul li {
	list-style-type: none;
	margin: 0;
	padding: 0;
	display: inline-block;
	float: left;
}

nav ul li a {
	display: block;
	color: #757575;
}

nav ul li ul li {
	float: none;
	display: block;
}

nav ul li a:hover{
	border-bottom: 5px solid #2b81eb;
}

nav ul li ul li a {
	padding: 10px 30px;
	background: #cccccc;
}
nav ul li ul li a:hover{
	border: none;
	padding: 10px 30px;
}

nav ul li ul li a:first-child(){
	background: url('');
}

And this is my JS:

jQuery(document).ready(function($){

    $( 'nav ul li a' ).hover(
        function(){
            $(this).children('ul.sub-menu').slideDown(200);
        },
        function(){
            $(this).children('ul.sub-menu').slideUp(200);
        }
    );
});