How to add custom invoice number like "INNK-5001" with auto increment using sqlite database in C# windows application

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C#

.NET

database

SQLite

I'm using SQLITE Database. I want to create a custom invoice number like this "INNK-5001" with auto increment INNK-5002, INNK-5003... and so on. I write below code for this purpose. Below code working correctly only if i use only number like "5000" but when i use "INNK-5000" i got Error. Please help me how can i get this number "INNK-5000" with auto increment using Sqlite Database. This my code...

What I have tried:

// To Create Sqlite Database

public void CreateDB()
    {
        if (!File.Exists("customid.db"))
        {
            SQLiteConnection.CreateFile("customid.db");
            using (SQLiteConnection conn = new SQLiteConnection("Data Source=customid.db;Version=3;"))
            {
                string commandstring = "CREATE TABLE cusid (Id INTEGER PRIMARY KEY NOT NULL, FirstName NVARCHAR(250), LastName NVARCHAR(250))";
                using (SQLiteCommand cmd = new SQLiteCommand(commandstring, conn))
                {
                    conn.Open();
                    cmd.ExecuteNonQuery();
                }
            }
        }
    }

// For Custom Invoice Number

public void CustomId2()
    {
        try
        {
            using (SQLiteConnection conn = new SQLiteConnection("Data Source=customid.db;Version=3;"))
            {
                string CommandText = "SELECT Id FROM cusid";
                using (SQLiteDataAdapter sda = new SQLiteDataAdapter(CommandText, conn))
                {
                    conn.Open();
                    DataTable datat = new DataTable();
                    sda.Fill(datat);

                    if (datat.Rows.Count < 1)
                    {
                        textBox1.Text = "INNK-5000";
                    }
                    else
                    {
                        using (SQLiteConnection conn1 = new SQLiteConnection("Data Source=customid.db;Version=3;"))
                        {
                            string CommandString = "SELECT MAX(Id) FROM cusid";
                            using (SQLiteCommand cmd = new SQLiteCommand(CommandString, conn))
                            {
                                conn1.Open();
                                int a = Convert.ToInt32(cmd.ExecuteScalar());
                                a = a + 1;
                                textBox1.Text = a.ToString();
                            }
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

// For Save a Record into Database

private void savebtn_Click(object sender, EventArgs e)
    {
        try
        {
            using (SQLiteConnection conn = new SQLiteConnection("Data Source=customid.db;Version=3;"))
            {
                string CommandText = "INSERT INTO cusid ([Id], [FirstName], [LastName]) VALUES (@id,@firstname,@lastname)";
                using (SQLiteCommand cmd = new SQLiteCommand(CommandText, conn))
                {
                    cmd.Parameters.AddWithValue("@id", textBox1.Text);
                    cmd.Parameters.AddWithValue("@firstname", textBox2.Text);
                    cmd.Parameters.AddWithValue("@lastname", textBox3.Text);

                    conn.Open();
                    int a = cmd.ExecuteNonQuery();
                    if (a > 0)
                    {
                        MessageBox.Show("Data Saved!!");
                        CustomId2();
                        BindDataGridView();
                    }
                    else
                    {
                        MessageBox.Show("Not Saved!!");
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

Please Help me... Thank You.