tamasnak

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <title>form</title>

    <meta name="description" content="Kovács Csaba">
    <meta name="author" content="Kovács Csaba">
</head>
<body>
<p id="price">...</p>
<form>
<input id="het" type="number" value="1">
</form>

<script>
var het =document.getElementById('het').addEventListener('change', function() {
    var price = document.getElementById('price');
    price.innerText = (this.value * 0.5);
});
</script>
</body>
</html>