  private void Div_Click(object sender, EventArgs e)
        {
            try
            {
                int a = Int32.Parse(textBox1.Text);
                int b = Int32.Parse(textBox2.Text);
                int r = a / b;
                label3.Text = r.ToString();
            }
            catch (FormatException)
            {
                MessageBox.Show("Format Exception ");
            }
            catch (DivideByZeroException)
            {
                MessageBox.Show("Divide By Zero Exception ");
            }
        }