Sonic Adventure 2 is a 2001 platform video game developed by Sonic Team USA and published by Sega.
 The sequel to "Sonic Adventure" (1998)"," it was the final "Sonic the Hedgehog" game for the Dreamcast after Sega discontinued the console.
 It features two good-vs-evil stories: Sonic the Hedgehog, Miles "Tails" Prower and Knuckles the Echidna attempt to save the world, while Shadow the Hedgehog, Doctor Eggman and Rouge the Bat attempt to conquer it.
 The stories are divided into three gameplay styles: fast-paced platforming for Sonic and Shadow, multi-directional shooting for Tails and Eggman, and action-exploration for Knuckles and Rouge.
Sonic Adventure is a platform video game developed by Sonic Team and published by Sega.
 Originally released for the Dreamcast, it was the first game in the main "Sonic the Hedgehog" series since "Sonic & Knuckles" (1994), and follows the respective quests of Sonic, Tails, Knuckles, Amy, E-102 Gamma, and Big to stop Doctor Ivo "Eggman" Robotnik from obtaining the seven Chaos Emeralds and releasing an ancient evil.
"Sonic the Hedgehog" is a video game series created by Yuji Naka.
 It is published by Sega, with entries developed by Sega, Sonic Team, Dimps, SIMS Co., Ltd., BioWare, and Sumo Digital.
 The series debuted in 1991 with the video game, "Sonic the Hedgehog", released for the Mega Drive video game console (named Genesis in North America).
 Most "Sonic the Hedgehog" games have either been platform games or released for Sega video game consoles and handheld game consoles (handhelds) dating from the Genesis to the eighth generation of video game consoles (2012–present).
 However, some of the original games were ported into versions on third-party home consoles and developed by several companies.
 As of February 2013, the series has collectively sold over 85 million copies worldwide across both the platform games and spin-offs.
Chao ( ; Japanese: チャオ ) are fictional life forms in the "Sonic the Hedgehog" video game series published by Sega.
 They are small, childlike creatures that go through a complex life cycle and exist in several visual forms depending on how they are raised.
 Developer Sonic Team incorporated Chao into the games to encourage players to explore levels and support the good–evil dichotomy of "Sonic Adventure 2".
Sonic Labyrinth (ソニックラビリンス , Sonikku Rabirinsu ) is a puzzle-platform game developed and published by Sega for the Game Gear, released in 1995.
 It is also playable on "Sonic Adventure DX" and on the PC, PlayStation 2 and Xbox editions of "Sonic Mega Collection Plus".
 The game features isometric 3D gameplay similar to that of the "SegaSonic the Hedgehog" (1993) arcade game and "Sonic 3D Blast" ("Sonic 3D: Flickies' Island" outside of North America), released one year later.
 Sonic, Dr. Eggman and Eggman's Robots are the only characters present in the game.
Tails' Skypatrol (テイルスのスカイパトロール , Teirusu no Sukaipatorōru ) is a scrolling shooter video game in the Sonic the Hedgehog series, developed by JSH and published by Sega for the Game Gear.
 It was originally released on April 28, 1995 exclusively in Japan, but was later released in other regions as an unlockable game in "Sonic Adventure DX" and "Sonic Gems Collection".
Twilight Syndrome is a horror themed visual novel / adventure video game series released exclusively in Japan.
 Goichi Suda directed the first two games and a spin-off, "Moonlight Syndrome".
Soldier of Fortune: Payback is a first-person shooter video game and the third installment of the "Soldier of Fortune" game series.
 Unlike the previous two "Soldier of Fortune" games, which were developed by Raven Software utilizing the id Tech 2 and id Tech 3, "Payback" was developed by Cauldron HQ, developed with Cauldron's in-house CloakNT engine, used in their previous first person shooter game, "Chaser".
 It is the first game of the series released for the Xbox 360 and PlayStation 3.
 The game was released on 14 November 2007.
Sonic the Hedgehog (Japanese: ソニック・ザ・ヘッジホッグ , Hepburn: Sonikku za Hejjihoggu ) is a video game franchise created and produced by Sega.
 The franchise centers on a series of speed-based platform games.
 The protagonist of the series is an anthropomorphic blue hedgehog named Sonic, whose peaceful life is often interrupted by the series' main antagonist, Doctor Eggman.
 Typically, Sonic—usually along with some of his friends, such as Tails, Amy, and Knuckles—must stop Eggman and foil any plans of world domination.
 The first game in the series, released in 1991, was conceived by Sega's Sonic Team division after Sega requested a mascot character.
 The title was a success, it spawned many sequels, and it transformed Sega into a leading video game company during the 16-bit era in the early to mid 1990s.
Chaos Rings is a role-playing video game series released primarily on mobile platforms.
 It is developed by Media.Vision and published by Square Enix.
 The eponymous first game in the series released for iOS in 2010, then later ported to other mobile and portable platforms including Android and PlayStation Vita.
 There are currently four games in the "Chaos Rings" series, and each uses the same gameplay base.
 Effective May 31, 2016, Square Enix ended distribution of the first three games in the series on Google Play and Apple's App Store, leaving Chaos Rings III as the only title still available for download or purchase on those markets.
 Currently, all titles in the series, except Chaos Rings III, are still available for purchase and download on Amazon's App Store.

Question: In which year was the first game of the Sonic Adventure video game series released?
Answer: 1991