private void btnInitialiseRandomNumbers_Click(object sender, EventArgs e)
        {
            var result = new List<int>();

            for (int i = 0; i <= 30; i++)
            {
                var rand = new Random();
                var randomNumber = rand.Next(1, 1000);

                var numberExists = listBox1.Items.IndexOf(randomNumber);

                while(numberExists != -1)
                {
                    randomNumber = rand.Next(1, 1000);
                    numberExists = listBox1.Items.IndexOf(randomNumber);
                }
                listBox1.Items.Add(randomNumber);
            }
        }