private void btnAdd_Click(object sender, EventArgs e)
        {
            //---create three variables for our data
            Single firstNumber  = 0;
            Single secondNumber  = 0;
            Single totalDue  = 0;
            

            //---assign the values
            firstNumber = Convert.ToSingle(txt1.Text);
            secondNumber = Convert.ToSingle(txt2.Text);
            totalDue = firstNumber + secondNumber;      //'add them
            Label1.Text = DoFormat(totalDue);               //let DoFormat function format as currency

        }

        private String DoFormat( Single x)
        {

            /* Private means it only works in this form & we pass in a single precision number called X &
                my function returns a string  */


            String myResult  = "";

            //---make 2 decimal places and show $   Use "n2" for number without $ and two decimals
            myResult = x.ToString("c2");
            return myResult;       //now send back the formatted string to the btnAdd_Click event so it can be shown in the label.

        }