###HUMAN: what are the cheapest round trip flights from DENVER to ATLANTA, ###ASSISTANT: SELECT DISTINCT flight.flight_id FROM airport_service AS AIRPORT_SERVICE_0, airport_service AS AIRPORT_SERVICE_1, city AS CITY_0, city AS CITY_1, fare, flight, flight_fare WHERE (CITY_0.city_code = AIRPORT_SERVICE_0.city_code AND CITY_0.city_name = 'DENVER' AND CITY_1.city_code = AIRPORT_SERVICE_1.city_code AND CITY_1.city_name = 'ATLANTA' AND flight.from_airport = AIRPORT_SERVICE_0.airport_code AND flight.to_airport = AIRPORT_SERVICE_1.airport_code) AND fare.round_trip_cost = (SELECT MIN(FAREalias1.round_trip_cost) FROM airport_service AS AIRPORT_SERVICEalias2, airport_service AS AIRPORT_SERVICEalias3, city AS CITYalias2, city AS CITYalias3, fare AS FAREalias1, flight AS FLIGHTalias1, flight_fare AS FLIGHT_FAREalias1 WHERE CITYalias2.city_code = AIRPORT_SERVICEalias2.city_code AND CITYalias2.city_name = 'DENVER' AND CITYalias3.city_code = AIRPORT_SERVICEalias3.city_code AND CITYalias3.city_name = 'ATLANTA' AND FLIGHT_FAREalias1.fare_id = FAREalias1.fare_id AND FLIGHTalias1.flight_id = FLIGHT_FAREalias1.flight_id AND FLIGHTalias1.from_airport = AIRPORT_SERVICEalias2.airport_code AND FLIGHTalias1.to_airport = AIRPORT_SERVICEalias3.airport_code) AND flight_fare.fare_id = fare.fare_id AND flight.flight_id = flight_fare.flight_id