private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            char ch = e.KeyChar;
            if ((ch == '+') || (ch == '-'))
                return;
            e.Handled = true;
        }

        private void maskedTextBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            char ch = e.KeyChar;
            if ((ch == '+') || (ch == '-'))
                return;
            e.Handled = true;
        }