private void btnmstr_Click(object sender, EventArgs e)
        {
            int z;
            z = Convert.ToInt32(sender.ToString().Substring(sender.ToString().Length - 1, 1));
            if ((editAnswr.Text == "0") || (op == true))
            {
                editAnswr.Text = Convert.ToString(z);
                op = false;
            }
            else
            {
                editAnswr.Text = editAnswr.Text + Convert.ToString(z);
            }
        }
    }