private void BtnGenerate_Click(object sender, EventArgs e)
        {
            ComboBox1.ResetText();
            ComboBox1.Items.Clear();
            Random r = new Random();

            for (int i = 0; i < r.Next(1, 500) - 1; i++)
            {
                ComboBox2.Items.Add(r.Next(100, 999));
            }
        }