Within each stage, the player advances by running, jumping, avoiding traps, shooting enemies and mini-bosses, and ultimately battling the Robot Master boss at the end. Mega Man begins with a "Mega Buster" arm-cannon weapon, but as each boss is defeated, a unique weapon is added to his arsenal. Because each Robot Master is weak to a specific weapon, the player may strategize the order in which the stages are cleared. The player also begins the game with the option to call on Mega Man's dog Rush, who can transform into a "Coil" springboard for jumping higher or a "Jet" for traversing long distances in the air. Mega Man's health, his special weapons, and his Rush options are all limited by energy gauges that can be refilled by picking up items found throughout each level. Special screws can also be picked up and used to buy items at Auto's shop between stages. These items include extra lives, tanks that refill energy, and one-time-use items that call on Mega Man's companions Eddie and Beat for help. Using the original Mega Man and Mega Man 2 as the main inspiration for the gameplay in Mega Man 9, several significant features from previous games were left out. Specifically, Mega Man lacks the ability to slide along the ground and the ability to charge up his Mega Buster for more powerful shots. Aside from using or omitting various elements from past games in the series, Mega Man 9 introduces a time attack mode and 50 optional challenges where certain requirements should be met to be accomplished. These range from defeating a boss using only the Mega Buster to clearing the entire game without taking any damage. Downloadable content for all three versions of the game was made available for purchase during October 2008. Proto Man can be downloaded as an alternate playable character. Proto Man is able to slide, can charge up his buster, and takes double the damage of Mega Man, among other differences. Additional downloadable add-ons include higher difficulty modes, an endless stage, and a special stage featuring a new boss, Fake Man. This downloadable content was included in Mega Man Legacy Collection 2, requiring the player to complete the game or enter a secret cheat code. Development Mega Man 9 was co-developed by Capcom and Inti Creates. The latter company had developed the Mega Man Zero and Mega Man ZX spin-off series