what is the cheapest round trip fare on continental 1291 round trip denver san francisco 
return flights ;return #1 that  are round trip ;return #2 on  continental 1291 ;return #3 from  denver ;return #4 to san francisco ;return fares of #5 ;return #6 that  is  the cheapest
['select', 'filter', 'filter', 'filter', 'filter', 'project', 'filter']