Adobe Acrobat Form : Adding 5 fields and allow "-" in any field

I want to add Five Fields (numeric) and also allow user to enter "-" (dash sign) if no value is present instead of 0.

but when i put "-" in any field it returns NaN.

Following is my script :

var n1 = Number(this.getField("Text1").valueAsString);
var n2 = Number(this.getField("Text2").valueAsString);
var n3 = Number(this.getField("Text3").valueAsString);
var n4 = Number(this.getField("Text4").valueAsString);
var n5 = Number(this.getField("Text5").valueAsString);

event.value = n1+n2+n3+n4+n5;

Thanks in advance

[moved by moderator to Acrobat from Community Help.]