Killer Sudoku Puzzle Books - 400 Easy to Master Puzzles 8x8 (Volume 2)

Killer Sudoku (also called "Sums Sudoku", "Sums Number Place", "Samunamupure", "Kikagaku Nampure") is a puzzle that combines elements of Sudoku and Kakuro. Despite the name, the simpler killer sudokus can be easier to solve than regular sudokus, depending on the solver's skill at mental arithmetic, the hardest ones, however, can take hours to crack. The puzzle is a 8x8 grid made up of 4x2 subgrids (called "regions"). In Killer Sudoku, sub-regions have specified sums, and no number can be repeated within a region.