I'm trying to make a simple php calculator which multiplies quantity to predefined prices. Here is my code but I got error 500. I can't manage to fix it by myself. It's html form with $_post checks and $_post submit button
// Check if form was submitted
$product = $_POST['product']; 
$quantity = $_POST['quantity'];
$first = 0,6;
$second = 0,5;
$third = 2,8;
if ( isset($_POST['submit']) ) {
    switch($_POST['product']) {
        case '1':
            $calculation = $quantity * $first;
            echo "Цена" . $calculation;
        break;
        case '2':
            $calculation = $quantity * $second;
            echo "Цена" . $calculation;
        break;
        case '3':
            $calculation = $quantity * $third;
            echo "Цена" . $calculation;
        break;
        default:
    }
}
?>
<form action="" method="post">
    <select name="product" id="product">
        <option>Choose servicve</option>
        <option value="1">Service 1</option>
        <option value="2">Service 2</option>
        <option value="3">Service 3</option>
    </select>
    <br>
    <input type="number" name="quantity" id="quantity">
    <br>
    <input type="submit" name="submit" value="submit">
</form><br /> ```
 
    