C# Adding Item at DataGridView using Datareader

I have a problem, I want to add an item using datareader in the datagridview which is I can only add once and if I add another one it'll error. Here's my codes:

connect = new OleDbConnection(coo); connect.Open();

        command.Connection = connect;
        DataTable dt = new DataTable();
        OleDbDataAdapter ODA = new OleDbDataAdapter("SELECT * FROM Items where itemno = '" + textBox1.Text + "'", connect);
        ODA.Fill(dt);

        command.CommandText = "SELECT * FROM Items";
        OleDbDataReader reader = command.ExecuteReader();
        object[] obj = new object[256];

        while (reader.Read())
        {
            reader.GetValues(obj);
            if ((textBox1.Text == obj[0].ToString()))
            {
                if (dataGridView1.DataSource == null)
                    dataGridView1.DataSource = dt;
                else
                    ((DataTable)dataGridView1.DataSource).Merge(dt);

                int sum1 = 0;
                for (int i = 0; i < dataGridView1.RowCount; i++)
                {
                    sum1 += Convert.ToInt32(dataGridView1.Rows[i].Cells[3].Value);
                }
                textBox2.Text = sum1.ToString();
                break;
            }
            else if (textBox1.Text==null)
            {
                break;
            }

        }
        reader.Close();
        connect.Close();

The error appeared as "Input string was not in a correct format" at the sum1 += Convert.ToInt32(dataGridView1.Rows[i].Cells[3].Value);... All I want is the items that I want to add at the datagridview have their prices and will add it all in the datagridview which is the total is in the textbox. plz help me guys..