Question: Refer to the Konami code, a sequence of 10 keystrokes using 6 different characters: ↑↑↓↓←→←→. This code was inadvertently left by the developer in the first release of a video game; when entered on the video game console as the title screen is open, it surreptitiously adds assets to the player's on-screen avatar. This code or versions of it have been retained in many video games.
How many 10-character codes can be created if only 2 down-arrow characters can be used and they must be paired together wherever they appear?