function calc(key)
    {
        var _key = document.getElementById(key);
        var _price_books = document.getElementById('price_books');
        var _price_books_total = document.getElementById('price_books_total');
        var _price = 0.0;
        var _price_total = 0.0;

//1-4 Bücher  Euro 12,80
//5-10    Bücher  Euro 9,80
//11-25   Bücher  Euro 7,80
//26  und mehr Bücher Euro 5,80

        if( _key.value == "" || _key.value <= 0 ) // if user de-selected the product...
        {
            var _price = 0.0;
            var _price_total = 0.0;
            _price_books.value = Number(_price);
            _price_books.value = Number(_price_total);
        }
        else
        {
            temp = _key.value;
            if (isNaN(temp))
            {
                alert("Error: Not a number!");
                _key.focus();
                return false;
            }
            if( temp < 5 )
            {
                _price = 12.8;
            }
            else if( 4 < temp && temp < 11 )
            {
                _price = 9.8;
            }
            else if( temp > 10 )
            {
                _price = 8.9;
            }
            //else if( 10 < temp && temp < 26 )
            //{
            //    _price = 7.8;
            //}
            //else if( temp > 25 )
            //{
            //    _price = 5.8;
            //}
            _price_total = ( _price * temp );
            _price_books.value = Number(_price);
            _price_books_total.value = Number(_price_total);
            return true;
        }// of else
    }// of function


    function Check_box(val)
    {
        var ch=document.getElementById('ch'+val);
        ch.checked=true;
    }