function update_total_price(id, amount) {

	var total_price;
	var total_price_hidden;
	var elm;
	var price;
	var comma;
	
	// get total price
	
	total_price = document.getElementById('total_price').innerHTML;	
	total_price_hidden = document.getElementById('total_price_hidden');

	elm = document.getElementById(id);
	
	if (elm.checked == true) {
		
		total_price = total_price.replace("€ ", "");
		total_price = total_price.replace(",-", "");
		total_price = total_price.replace(",", ".");

		price = Number(total_price) + (Number(amount));
		price = Math.round(price * 100) / 100;
		price = String(price);
		
		// set hidden field
		total_price_hidden.value = price;
		
		comma = price.indexOf(".");

		if (comma != -1) {
		
			// replace "." with ","
			price = price.replace(".", ",");
		
		} else {
			
			price = price + ",-";
			
		}
		
		document.getElementById('total_price').innerHTML = "€ " + price;
		
	} else {
		
		total_price = total_price.replace("€ ", "");
		total_price = total_price.replace(",-", "");
		total_price = total_price.replace(",", ".");	

		price = Number(total_price) - (Number(amount));
		price = Math.round(price * 100) / 100;
		price = String(price);
		
		// set hidden field
		total_price_hidden.value = price; 
		
		comma = price.indexOf(".");

		if (comma != -1) {
		
			// replace "." with ","
			price = price.replace(".", ",");
		
		} else {
			
			price = price + ",-";
			
		}

		document.getElementById('total_price').innerHTML = "€ " + price;
		
	}
}