Hi, Here is the situation/question:

I'm gonna be in Istanbul for 3 months this summer. A round-trip flight from Chicago to Istanbul (departure:June, return:September) is $1950.
But if I add another round-trip from Istanbul to -say- London within this 3 month period, the price goes down to $1200. For example,

1. June 11 - CHI-IST
2. July 11 - IST-LON
3. July 25 - LON-IST
4. Sep 18 - IST-CHI

This whole multi-city ticket is much cheaper. Note that there are no connecting flights or layovers. So, if I don't go to London from Istanbul (That is, skip flights 2 and 3), do you think I can still take the flight back from Istanbul to Chicago in September? Or, would the airline consider this as "hidden city ticketing" and cancel my return ticket (ticket 4)?

Thanks!