Script to calculate via action button

I have two fields .. field1 and field2 in which value will be entered and a 3rd field....... field3 which will be equal to field 1 + field 2 .

I want the calculation to be performed when i click a action button say "button1"

following was the script i used but it;s now working. I put this script in the Action button's "run a javascript"

this.getField("field3").value = this.getField("field1").value + this.getField("field2").value;

Plz help