Half-Life 2 (stylized as HλLF-LIFE2; often shortened to HL2 or λ2), the sequel to Half-Life, is a first-person shooter video game and a signature title in the Half-Life series. Developed by Valve Corporation, it was initially released on November 16, 2004, following a protracted five-year, $40 million development cycle, during which a substantial part of the project was leaked and distributed on the Internet. The game was developed alongside Valve's Steam software. It introduced the Source game engine and, because of Steam, is the first video game to require online product activation. Like its predecessor, Half-Life 2 was met with near-unanimous critical acclaim. It was praised for its advanced physics, animation, sound, AI, graphics, and narrative. The game won 39 "Game of the Year" awards, and a couple of publications have named it "Game of the Decade". It won the title of "Game Of The Decade" at the 2012 Spike Video Game Awards. Over 6.5 million copies of Half-Life 2 were sold at retail by December 3, 2008, making it a bestselling PC game (not including the number of sales via Steam). As of February 9, 2011, Half-Life 2 has sold over 12 million copies.