Boxing, also called pugilism, is a combat sport in which two people fight each other using their fists. Boxing is supervised by a referee over a series of between one to three minute intervals called rounds. The boxers are generally of similar weight. There are four ways to win; if the opponent is knocked out and unable to get up before the referee counts to ten seconds (a knockout, or KO), if the opponent is deemed unable to continue (a Technical Knockout, or TKO), if an opponent is disqualified for breaking a rule, or a winner is determined either by the referee's decision or by judges' scorecards at the end of the bout. The birth hour of boxing as a sport may be its acceptance by the ancient Greeks as an Olympic game as early as 688 BC. Modern boxing evolved in Europe, particularly Great Britain.