count all those whose youthclub performed at the 2013–14 algerian ligue professionnelle 1 ?
SELECT DISTINCT COUNT attr_open var_uri attr_close where brack_open var_x dbo_season dbr_2013–14_Algerian_Ligue_Professionnelle_1 sep_dot var_uri dbp_youthclubs var_x sep_dot var_uri a dbo_Athlete brack_close