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Hot on the heels of Sudoku comes Kakuro. Think of them as the mathematical equivalent of crosswords. Also known as Cross Sums or Kakro in Japan . A Kakuro consists of a playing area of filled and empty cells similar to a crossword puzzle. Some black cells contain a diagonal slash from top left to bottom right with numbers in them, called "the clues". A number in the top right corner relates to an "across" clue and one in the bottom left a "down" clue.