private void Code_Click(object sender, EventArgs e)
{
  int num = System.Convert.ToInt32(textBox1.Text, 10);
  if (num >= 1000 && num <= 9999)
  {
     int sum = 0, base1 = 1, n = num;
     while (n > 0)
     {
         sum += (n % 10 % 4 + 1) * base1;
         base1 *= 10;
         n /= 10;
      }
      int n1 = sum % 10;
      int n3 = sum / 100 % 10;
      sum -= (n1 + n3 * 100);
      sum += (n3 + n1 * 100);
      textBox2.Text = sum.ToString();
  }
  else
      textBox2.Text = "Error";
}
