How to handle input from a textbox to not print a NaN

I am trying to get user input from the textbox and then click the button beside in order to calculate the area of a circle. Problem is alert always prints NaN.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>HTML 5 Boilerplate</title>
    <link rel="stylesheet" href="./css/style.css">
    <script>
        const PI = 3.14159;

        function getAreaOfCircle(rad) {
            let area = PI * Math.pow(rad, 2);
            alert(area);
        }
    </script>
</head>
<body>
    <input type="text" placeholder="Type something..." id="myInput">
    <input type="button" onclick="getAreaOfCircle();" value="Calculate Area of a Circle">
</body>
</html>