La moyenne dans datagridview

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            for (int i = 0; i < 4; i++)
            {
                DataGridViewColumn dt = new DataGridViewColumn(new DataGridViewTextBoxCell());
                DataGridViewRow row = new DataGridViewRow();
                dt.HeaderText = "Value" + i;
                dt.Name = dt.HeaderText;
                dataGridView1.Columns.Add(dt);
                dataGridView1.Rows.Add(row);
                if (i<3)
                    dataGridView1.CellValueChanged+=new DataGridViewCellEventHandler(OnChanged);
            }

        }
        private void OnChanged(object sender, DataGridViewCellEventArgs e)
        {
            bool ok=true;
            for (int i = 0; i < 3; i++)
                ok &= (dataGridView1[i, e.RowIndex].Value != null);
            if (ok)
            {
                double moyenne=0;
                for (int i = 0; i < 3; i++)
                    moyenne += Convert.ToDouble(dataGridView1[i, e.RowIndex].Value);
                dataGridView1[3, e.RowIndex].Value = Math.Round(moyenne / 3,3);
            }

        }

    }