Button is Disable when textbox is Empty

I'm using VS 2010.

     I have three text boxes in my application (txt1,txt2,txt3) and a button(btnSubmit). I try to calculate first two text boxes and display the result in third textbox automatically.

See My code Below:

page.aspx

<script type="text/javascript">
    function calc() 
            {
            var sum = 0;
            sum = parseInt(document.getElementById('txt1').value)
                + parseInt(document.getElementById('txt2').value)
                document.getElementById('txt3').value = sum;
            }
     </script>

 page.aspx.cs

protected void Page_Load(object sender, EventArgs e)
    {
        btnSubmit.Enabled = false;
    }

      The Problem is I write the code to disable the button(btnSubmit) when page is load. After the calculation result is display in third textbox the button(btnSubmit) will Enable. Tell me how?

Help Me....

Thanks in advance.......