index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
    <div class="mid">
                <div class="welcome">Mailes converter</div>
                <input type="number" name="" id="data" step="any"/> <p>Kms</p> <br><br>
                <button onclick="convert()">Convert</button>

                <h1 id="content" ></h1>
    </div> 



    <script>
        function convert(){
            var kms = document.getElementById('data').value;
            const factor = 0.621371;
            var mails = kms *factor;
            
            document.getElementById('content').innerText = mails + ' mails';
    }
    </script>
</body>
</html>