LONDON — The finale was a formality, more a coronation than a contest. Michael Phelps headed into the retirement the only way imaginable with another gold medal.Reclaiming the lead with his trademark butterfly stroke, the one most people first saw in Sydney a dozen years ago, Phelps won the 18th gold of a mind-boggling career in the 4x100-meter medley relay Saturday.When it was done, Phelps hugged his teammates Matt Grevers, Brendan Hansen and Nathan Adrian before heading off the deck for the final time in his suit. He waved to the crowd and smiled, clearly at peace with his decision to call it a career.And what a career it was!Phelps retires with twice as many golds as any other Olympian, and his total of 22 medals is easily the best mark, too. He can be quite proud of his final Olympics as well, even though there were times he had trouble staying motivated after winning a record eight gold medals at the Beijing Games four years ago.Bouncing back from a disappointing first race in London, a fourth-place finish in the 400 individual medley, Phelps wound up with more medals than any other swimmer at the games: four golds and two silvers.Sounds familiar.Grevers had the Americans in front on the opening backstroke leg of the relay, but Kosuke Kitajima put Japan slightly ahead going against Hansen in the breaststroke. Not to worry, not with Phelps going next. He surged through the water, handing off a lead of about a quarter of a second to Adrian for the freesytle anchor. The Americans won going away in 3 minutes, 29.35 seconds, just off their own Olympic record from Beijing. Japan held on for silver in 3:31.26, with Australia taking the bronze in 3:31.68.