// JavaScript Document

function countPrice(which_field, cb, total_field) 
{
	
	var p = parseInt(document.getElementById(which_field).innerHTML);
	var t = parseInt(document.getElementById(total_field).innerHTML);
	
	if (cb.checked)
		t += p;
	else
		if (t > p)
			t -= p;
		else
			t = 0;
	
	document.getElementById(total_field).innerHTML = t;
	
}