please list all flights on united airlines and northwest airlines that go into denver and all flights on northwest airlines and united airlines that fly out of denver
O O O O O B-airline_name I-airline_name O B-airline_name I-airline_name O O O B-toloc.city_name O O O O B-airline_name I-airline_name O B-airline_name I-airline_name O O O O B-fromloc.city_name