Sum If Less Than Zero in PDF

Sum of A+B should be less than 0 and if it is then show result in 3rd field?

You can use this in 3rd field as 'Custom calculation script':

var A = Number(this.getField("Text Field A").valueAsString);
var B = Number(this.getField("Text Field B").valueAsString);

var total = A+B;

if(total < 0)
event.value = total;
else
event.value = "";