private void button1_Click(object sender, EventArgs e)
        {
            string Key = textBox1.Text.ToString();

            if (textBox1.Text == null)
            {
                MessageBox.Show("Mật khẩu chưa nhập! ", "Thông báo", MessageBoxButtons.OK);
            }
            if (int.Parse(Key) % 2 == 0)
            {
                using (MD5 md5 = MD5.Create())
                {
                    string hash = GetMd5Hash(md5, Key);
                    System.IO.StreamWriter str = System.IO.File.AppendText("user.txt");
                    str.WriteLine(hash);
                    str.Close();
                    this.Hide();
                    StartScreen startScreen = new StartScreen();
                    startScreen.ShowDialog();
                    this.Close();
                }
            }
            else
            {
                MessageBox.Show("Sai mật khẩu! ", "Thông báo", MessageBoxButtons.OK);
            }
        }
        private void KeyLogin_Load(object sender, EventArgs e)
        {
            if (!System.IO.File.Exists("user.txt"))
            {
                var myfile = System.IO.File.Create("user.txt");
                myfile.Close();
            }
            string pass = System.IO.File.ReadAllText("user.txt");

            if (pass.Length != 0)
            {
                this.Hide();
                StartScreen startScreen = new StartScreen();
                startScreen.ShowDialog();
                this.Close();
            }
        }