var auth = new OAuth2Authenticator (
            clientId: "Id клиента, который запомнили на прошлом шаге",
            scope: "friends,video,groups",
            authorizeUrl: new Uri ("https://oauth.vk.com/authorize"),
            redirectUrl: new Uri ("https://oauth.vk.com/blank.html"));
        auth.AllowCancel = true;
        auth.Completed += (s, ee) => {
            if (!ee.IsAuthenticated) {
                var builder = new AlertDialog.Builder (this);
                builder.SetMessage ("Not Authenticated");
                builder.SetPositiveButton ("Ok", (o, e) => { });
                builder.Create().Show();
                return;
            }