I need to know row index of selected row so I can use it on server side.

On my grid I have this:

   <ext:GridPanel ID="GridPanel1" runat="server" StripeRows="true" Title="EmQ"
                    Icon="UserSuit" Visible="true" AutoDataBind="True" Region="Center" Frame="true"">
                    <ColumnModel ID="ColumnModel1" runat="server">
                            <ext:Column Header="First Name" DataIndex="FN"></ext:Column>
                            <ext:Column ColumnID="ID" Header="Last Name" DataIndex="LN"></ext:Column> 
                                <ext:RowSelectionModel ID="RowSelectionModel1" runat="server" SingleSelect="true">
                                        <RowSelect Handler="#{FormPanel1}.getForm().loadRecord(record);" />         
and on button (event when i edit selected row) I have this:
     <ext:Button ID="btnEdit" runat="server" AutoPostBack="true" Text="Edit" Region="Center" Width="60" Height="25" OnClick="btnEdit_OnClick">
                          <Click Handler="#{GridPanel1}.getStore().reload();" />                          
Can you show me how can i get row index of selected row? I suppose I need to put ExtraParams on Button/Listener..?i'm new in this, and I'll appreciate if you give me solution for this.