private void Search_Click(object sender, EventArgs e)
{
  int[] a = new int[20] { 1, 10, 15, 17, 25, 30, 35, 45, 50, 52, 63, 73, 80, 82, 85, 87, 92, 94, 96, 98 };
  int num = System.Convert.ToInt32(textBox1.Text, 10);
  bool find = false;
  int low = 0;
  int high = 19;
  while (high >= low && !find)
  {
      int mid = (int)(high + low) / 2;
      if (a[mid] == num)
         find = true;
      else if (num > a[mid])
         low = mid + 1;
      else high = mid - 1;
  }  
  if (find)
     label2.Text = "Found";
  else
    label2.Text = "Not Found";
}
