What is the relationship between medium and San Francisco in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

SAN FRANCISCO, Sept 25 Video gamers who live-stream their matches online and the throngs of fans who tune in daily to watch them swarmed a first-of-its kind convention in San Francisco on Friday, as interest swells in the medium of online video game broadcasting.

Answer: headquarters_location