static void WriteFile(string filename, string text)
        {
            File.WriteAllText(filename, text);
        }

        static string ReadFile(string filename)
        {
            return File.ReadAllText(filename);
        }

        private void btnAddNr_Click(object sender, EventArgs e)
        {
            string text = txtInput.Text;

            WriteFile(filename, text + Environment.NewLine);
        }

        private void btnGetMean_Click(object sender, EventArgs e)
        {
            string text = ReadFile(filename);

            string[] lines = text.Split(new string[] { Environment.NewLine }, StringSplitOptions.None);

            int summa = 0;

            foreach (string line in lines)
            {
                 summa += Convert.ToInt16(line);
            }


        }