private void btnPoint_Click(object sender, RoutedEventArgs e)
 {
     if (Result_txt.Text == "Enter Equation" || Result_txt.Text == "UNDEFINED")
     {
         Result_txt.Text = ".";
     }
     else
     {
         Result_txt.AppendText(".");
     }
 }
        private void btnMod_Click(object sender, RoutedEventArgs e)
        {
            if (operation != "")
            {
                MessageBox.Show("Operator already present");
            }

            if (Result_txt.Text == "Enter Equation" && operation == "")
            {
                Result_txt.Text = "%";
                operation       = "%";
            }
            if (operation == "" && Result_txt.Text != "UNDEFINED")
            {
                Result_txt.AppendText("%");
                operation = "%";
            }
        }