Nan problem in javascript calculation

See more:

C#

ASP.NET

Javascript

MVC3

i am getting value as NaN.when i try to blank my textbox .why i am getting this.

What I have tried:

THIS IS MY SCRIPT:

$("#txtSC").keyup(function () {


var myBox1 = document.getElementById('box1').value;
var myBox2 = document.getElementById('box2').value;
var mybox3 = document.getElementById('box3').value;
var mybox4 = document.getElementById('txtDiscount').value;
var mybox5 = document.getElementById('txtSC').value;

var multply = myBox1 * myBox2;
var per = (multply * mybox3) / 100;
var myResult = multply + per;
var withDisCount = (myResult * mybox4) / 100;
var myDiscount = myResult - withDisCount;
var ShippingCost = myDiscount + (parseFloat(mybox5));

// var finalResult = Math.round(ShippingCost);
var result = document.getElementById('TotalAmt');

result.value = ShippingCost;




});


on last textbox i.e mybox5, when i am trying to backspace in that textbox(making it empty manually) my "TotalAmt" textbox show NaN value.