How To Get Domain non built-in Groups using C#?

i have a c# function that gives me all of domain group's name :

public static List<String> ListAllDomainGroups()
{
    List<String> groups = new List<string>();

    // create your domain context
    PrincipalContext ctx = new PrincipalContext(ContextType.Domain);

    // define a "query-by-example" principal - here, we search for a GroupPrincipal 
    GroupPrincipal qbeGroup = new GroupPrincipal(ctx);

    // create your principal searcher passing in the QBE principal    
    PrincipalSearcher srch = new PrincipalSearcher(qbeGroup);

    // find all matches
    foreach (var found in srch.FindAll())
    {

        // do whatever here - "found" is of type "Principal" - it could be user, group, computer.....          
        //listBox2.Items.Add(found.ToString());
        groups.Add(found.ToString());
    }

    return groups;
}

function returns about 70 names including :

WinRMRemoteWMIUsers_

Administrators

Users

Guest

Print Operations

Backup Operators

Replicator

IIS_IUSRS

Remote Desktop Users

... ...

(then groups i defined)

how can i get only the groups that i have defined in my AD and not the built-in groups ?