Calculate Circle Area

<!--Write a HTML page that includes the script and calculates and prints in the page body the area of circles of size r=7, r=1.5 and r=20.-->

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta charset="utf-8" />
    <title>Circle Area</title>
    <style type="text/css">
        body p {
            font-size: 20px;
            margin: 0;
        }
    </style>
</head>
<body>
    <p>r = 7; area = </p>
    <p>r = 1.5; area = </p>
    <p>r = 20; area = </p>
</body>
<script type="text/javascript">
    "use strict";

    function calcCircleArea(r) {
        return Math.PI * r * r;
    }

    var elements = document.getElementsByTagName("p");
    for (var index in elements) {
        var item = elements[index];
        var r = parseFloat(item.innerHTML.substr(4, 7));
        item.innerHTML += calcCircleArea(r);
    }
</script>
</html>