Reset serial (row) number in GridView based on condition using C# and VB.Net in ASP.Net

I want to display row number in column0 of gridview with condition if column1 = "" do nothing and start count form 1 again

col0 col1

1      a

2      a

3      a

4      a

1     

2      a      

1     

2      a

3      a

4      a

    Private Sub GridView1_RowDataBound(sender As Object, e As GridViewRowEventArgs) Handles GridView1.RowDataBound
        Dim m As Integer
        If e.Row.Cells(1).Text = "" Then

            m = e.Row.RowIndex + 1
        Else
            e.Row.Cells(0).Text = (i + 1 - m).ToString

        End If

        If e.Row.Cells(0).Text = "0" Or e.Row.Cells(0).Text = "-1" Then
            e.Row.Cells(0).Text = ""
        End If
   End Sub