How can I sum a particular column of all selected rows? (Via javascript)

            
    <script>
        var sumCheck= function () {
            var resultSum;

          /* todo loop selectedrows
             resultSum += column(nvlparmvfin); */;
            
            alert(resultSum);
        };
    </script>
       
 <ext:GridPanel runat="server" ID="grid1" Height="250px" Frame="true" StoreID="dsGrid1">
                        <ColumnModel runat="server">
                            <Columns>
                                <ext:NumberColumn runat="server" DataIndex="nvlparmvfin" Format=",0.00" Align="Right"/>                             
                            </Columns>
                        </ColumnModel>
                        <SelectionModel>
                            <ext:CheckboxSelectionModel runat="server" Mode="Multi">
                                <Listeners>
                                    <SelectionChange Fn="sumCheck()" />
                                </Listeners> 
                            </ext:CheckboxSelectionModel>
                        </SelectionModel>                       
</ext:GridPanel>
If you can help me, thank you very much;