last value of two groups of radio buttons

Hi,

I have two radio buttons DS fiels (database source). One is fieldname4 and the other is fieldname5. Both of them have class .special-radio.

I need to know what the value of the last selected input of the two groups is and that this vaule . For example:
My client press a radio button of fieldname4, then in one calculate field I make operatios with this value. But if my client press in the other radio buttons Ds field (fieldname5), this new value must be recognized by the calculate field and recalculate.

I have this function:
(function(){
var values = [fieldname4,fieldname5],
total = 0,
counter = 0;
for(var i in values)
{
if(values[i]) counter++;
total += values[i];
}
var result = PREC((counter)?total/counter:0, 0);
return result;
})()

The problem is that if my client press three o four times other radio buttons, the forms stop working.

Best regards