Refer to the passage below and then answer the question afterwards in the same language as the passage:

Passage: Cholesterol gallstones can sometimes be dissolved with ursodeoxycholic acid taken by mouth, but it may be necessary for the person to take this medication for years. Gallstones may recur, however, once the drug is stopped. Obstruction of the common bile duct with gallstones can sometimes be relieved by endoscopic retrograde sphincterotomy (ERS) following endoscopic retrograde cholangiopancreatography (ERCP).

Question: What is one possible treatment for gallstones?
endoscopic retrograde sphincterotomy