question: what is the cheapest round trip fare on continental 1291 round trip denver san francisco 
#1 return flights 
#2 return #1 that  are round trip 
#3 return #2 on  continental 1291 
#4 return #3 from  denver 
#5 return #4 to san francisco 
#6 return fares of #5 
#7 return #6 that  is  the cheapest