private void OperationButtonClick(object sender, EventArgs e)
{
var s = (sender as Button).Text;
var s1 = BOX.Text.Substring(0, BOX.SelectionStart);
var s2 = BOX.Text.Substring(BOX.SelectionStart + BOX.SelectionLength,
BOX.Text.Length - BOX.SelectionStart - BOX.SelectionLength);
BOX.Text = s1 + s + s2;
BOX.SelectionStart = s1.Length + s.Length;
BOX.SelectionLength = 0;
BOX.Focus();
}