Calcudoku Plus Minus Puzzle Books: Calcudoku Plus Minus Puzzle Books - 400 Easy to Master Puzzles 8x8 (Volume 4) (Paperback)

Calcudoku (also known as "Mathdoku") is a mathematical and logical puzzle loosely similar to Sudoku.
The objective is to fill the grid in with the digits 1 through N (where N is the number of rows or columns in the grid) such that:
- Each row contains exactly one of each digit.
- Each column contains exactly one of each digit.
- Each bold-outlined group of cells (block) contains digits which achieve the specified result using the specified mathematical operation: addition (+) and subtraction (-).
- Unlike Killer Sudoku, digits may repeat within a block.