sum of values in textboxes

Hi,

I have 3 textboxes  want to add values in two textboxes into  third textbox  without button click.

c=a+b;

    protected void txt_Mon_TextChanged(object sender, EventArgs e)
    {
        if ((!string.IsNullOrEmpty(txt_Mon.Text)) && (!string.IsNullOrEmpty(txt_Tue.Text)))
        {
            txt_Total.Text = (Convert.ToInt32(txt_Mon.Text) + Convert.ToInt32(txt_Tue.Text)).ToString();
        }

    }