function change_value(element)
			{
			  check_value = element.value;
			  switch (check_value)
				{
				  case "":
				  case " ":
				  case "00":
				  case "000":
					  element.value = 0;
					  break;
				  default :
				      break;
				}
			  if (isNaN(check_value))
			    {
				  alert('Por favor ingrese un valor numerico');
				  eval(element.value = 0);	
			    }	
			  else
			    {
				  if (check_value<0)
				    {
				      alert('Por favor ingrese un valor positivo');
					  eval(element.value = 0);			
					  return false;
				    }
				}
			  element.value = eval(element.value);
			}
		  function total_calc()
		    {
			  //alert(arguments.length);
			  total = 0;
			  for (i=1; i<arguments.length; i++)
			    {
				  valor = document.getElementById(arguments[i]).value;
				  total += eval(valor);
				}
			  document.getElementById(arguments[0]).value = total;
			}