Each level starts with the quantity, types, and firing order of birds predetermined. If all of the pigs are eliminated after the last bird is launched, the level is cleared and the next level is unlocked. If all of the birds run out and the pigs are not defeated, the level is failed and must be repeated. Points are scored for each pig defeated as well as for damage to, or destruction of, structures, and hefty bonus points are awarded for any unused birds. Upon completing each level, players can receive up to three stars depending on the score received. Players are encouraged to repeat any previously beaten levels in which the full three stars weren't awarded in order to fully master them and earn the full three star rating. Development In early 2009, Rovio was in a state of bankruptcy; the staff began viewing proposals for potential games. One such proposal came from senior game designer Jaakko Iisalo in the form of a simulated screenshot featuring some angry-looking birds with no visible legs or wings. While the picture gave no clue as to what type of game was being played, the staff liked the characters, and the team elected to design a game around them. In early 2009, physics games, such as Crush the Castle, were popular flash-based web games, so the Rovio team was inspired by these games. During the development of Angry Birds, the staff realized the birds needed an enemy. At the time, the "swine flu" epidemic was in the news, so the staff made the birds' enemies pigs. Angry Birds was the studio's 52nd produced game and on its initial release, the game did not sell many copies. After Angry Birds was a featured app on the UK App Store in February 2010 and quickly reached No. 1 there, it reached the No. 1 spot on the paid apps chart in the US App Store in the middle of 2010, staying in that milestone for 275 days. The initial cost to develop Angry Birds was estimated to exceed €100,000, not including costs for the subsequent updates. In terms of publishing for the iOS version, Rovio partnered with distributor Chillingo to publish the game to the App Store. Chillingo claimed to have participated in final game polishing, such as adding visible trajectory lines, pinch to zoom, pigs' grunts, birds' somersaults on landing. Since then Rovio has