Simplify the following text for a Grade 8 native English-speaking person.
Original: PokÃ mon Mystery Dungeon : Explorers of Time and PokÃ mon Mystery Dungeon : Explorers of Darkness are a the sequel to the PokÃ mon Mystery Dungeon : Blue Rescue Team and Red Rescue Team games . It was released in Japan on September 13 , 2007 , in North America on April 20 , 2008 , in Europe on July 4 , 2008 and in Australia on June 19 , 2008 .
Simplified: As of September 30 , 2008 , PokÃ mon Mystery Dungeon : Explorers of Time and Explorers of Darkness combined to sell 4.12 million copies worldwide , according to Nintendo .