подскажите где ошибка, не срабатывает аккордеон
<!DOCTYPE html>
<html lang="ru">
<head>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap-theme.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
</head>
<body>
<div class="panel-group" id="collapse-group">
<div class="panel panel-default">
<div class="panel-heading">
<h4 class="panel-title">
<a data-toggle="collapse" data-parent="#collapse-group" href="#el1">Первый элемент</a>
</h4>
</div>
<div id="el1" class="panel-collapse collapse in">
<div class="panel-body">Lorem ipsum dolor sit amet, consectetur adipisicing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</div>
</div>
</div>
<div class="panel panel-default">
<div class="panel-heading">
<h4 class="panel-title">
<a data-toggle="collapse" data-parent="#collapse-group" href="#el2">Второй элемент</a>
</h4>
</div>
<div id="el2" class="panel-collapse collapse">
<div class="panel-body">Lorem ipsum dolor sit amet, consectetur adipisicing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</div>
</div>
</div>
<div class="panel panel-default">
<div class="panel-heading">
<h4 class="panel-title">
<a data-toggle="collapse" data-parent="#collapse-group" href="#el3">Третий элемент</a>
</h4>
</div>
<div id="el3" class="panel-collapse collapse">
<div class="panel-body">Lorem ipsum dolor sit amet, consectetur adipisicing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</div>
</div>
</div>
</div>
</body>
</html>
