numeric help

Im trying to add two form fields as a number its not working, its just
adding them together as a string and not performing mathmatics on them

any suggestions

my code (it maybe wrong)

x = form1.cost1.value;
y = form1.cost2.value;
var x;
var y;
form1.grandtotal.value = (x + y)


cost1 & cost2 both being my fields within the form

Cheers

Rob