为什么不使用高斯公式.(N*(N+1))/2

Why not use Gauss formula. (N*(N+1))/2

private void btnAddAll_Click(object sender, EventArgs e)
{
     int n, answer;  
     n = int.Parse(txtNum.Text);
     answer = (n*(n+1))/2;
     lstShow.Items.Add("Sum = " + answer);
     lstShow.Text = answer.ToString();
}