The logic of Sudoku is combined with the math play of KenKen(R) to form Kendoku. The rules are simple enough, the results are devastatingly difficult: The puzzles, here all 9x9, look like Killer Sudoku games but in addition to numbers, players also must contend with math operators (+ - ) within "fences" on the puzzle. And in this book, only the first round of puzzles display the operators. The total at the top left of the fenced area is arrived at by using the operator on all the numbers within the fenced area. The solution is a valid Sudoku solution. But unlike in Killer Sudoku, in Kendoku numbers are allowed to be repeated within a fenced area as long as the result doesn't break the rules for a valid Sudoku.
Don't even think about using a pen to solve these brain-jarringly tough puzzles that move Kendoku beyond extreme.