I am collecting a text area value(number in decimal ) using its id to a javascript variable, adding another variable value (number in decimal), and displaying the result in same html page

text area code:

<div class=input-resp><span><input  class=textbox id=num name=count type=text size=5 maxlength=3  value= /></span></div>

passing value to var and adding it to another var

var a = document.getElementById('num').value;

var b = 1000;

var c = parseFloat(a) + parseFloat(b); 

passing var c to div id test

document.getElementById(test).innerHTML = c;

html for test id

<div id=test></div>

But here, whenever I am updating the text area with new number, the final output is not updating instantly.I have refresh the page manually to get new value. Is there anyway I update the value without a page refresh ?