vertical toggle for asp submenu items

Question

I'm working on asp menu with vertical orientation and trying to toggle the submenu items in same list.

I want to achieve something like this.

and right now i've submenu in extended list like this.

Following is my code.

<asp:Menu ID="NavigationMenu" runat="server" StaticDisplayLevels="1"  
    RenderingMode="List" DataSourceID="SiteMapDataSource1"  CssClass="menu-item-parent"                              
    OnMenuItemDataBound="NavigationMenu_MenuItemDataBound"
    OnDataBound="NavigationMenu_DataBound">
</asp:Menu>

I am not sure how can i display the submenu in same list. can please someone suggest a solution? Thanks in advance.