I need help with math script

Question

here's what i want to do.
chart looks like this:
number 1
-/+
number 2
number 3
number 4
total
if '+' sign or no sign is used in the space for it, number1 + number2 + number3 + number4 = total
if '-' sign is used, then number1 - number2 + number3 + number4 = total
any suggestions? i tried this;
var a = this.getField ("2"); 'corresponds to number 1'
var b = this.getField ("3"); 'corresponds to number 2'
var c = this.getField ("4"); 'corresponds to number 3'
var d = this.getField ("5"); 'corresponds to number 4'
var e = this.getField ("7"); 'corresponds to number -/+'
if (e.value = "-"){
event.value = a.value - b.value + c.value + d.value;
else {
event.value = a.value + b.value + c.value + d.value;
}

Answer 1

Ok, thanks for the replies, and I will try the suggestions, but I finally solved the problem. Here it is.
var a = this.getField ("2");
var b = this.getField ("3");
var c = this.getField ("4");
var d = this.getField ("5");
var e = this.getField ("7");
if (e.value == "-")
event.value = a.value - b.value + c.value + d.value;
else
event.value = a.value + b.value + c.value + d.value;