Simple Addition Using Integers Stored In Variables

i have this simple function...

function update(value, oldvalue)
{
var a = value;
var b = oldvalue;
var result = a + b;
document.form.fieldname.value=result;
}

and call the function using a field with this...

onkeyup="update(this.value, other)"

But all its doing is concatenating the 2 numbers together as though they are strings!!!

so if this.value = 11
and other = 15
it displays 1115

i want to add them and display the total, do i need to specify them as being integers or something?