How can I sum datagrid same values in group

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim data = firstDgv.Rows.Cast(Of DataGridViewRow)() _
         .GroupBy(Function(r) New With {Key .A = r.Cells(0).Value, Key .B = r.Cells(1).Value}) _
         .Select(Function(grp) New With
         {
            grp.Key.A,
            grp.Key.B,
            .C = grp.Sum(Function(x) CInt(x.Cells(2).Value))
         }) _
         .ToList()

    For Each dgr In data
        secondDgr.Rows.Add(New Object() {dgr.A, dgr.B, dgr.C})
    Next
End Sub