How do i stop concatenation in my function? (java script)?

[Alexander R]

<script type="text/javascript">


function calculate_total(orderForm) {

var counter=0;
var orderForm=document.getElementById(orderForm);

if(orderForm.DigRacing.checked){
counter = counter+(orderForm.DigRacing.value);
}
if(orderForm.AtariJoystick.checked){
counter = counter+(orderForm.AtariJoystick.value);
}
if(orderForm.FXLightsaber.checked){
counter = counter+(orderForm.FXLightsaber.value);
}

orderForm.total.value='\u00A3' + counter;
}


</script>