Killer Sudoku

Killer Sudoku (Sum Sudoku, Kikagaku Nampure) is a variant of the classic Sudoku puzzle that incorporates elements of Kakuro.

Rules: In each cell of the NxN grid, enter a number in the range 1–N. Each number must appear exactly once in each row and column, and in each grey and white block (but not in each region with a given number).
Additional rules: The sum of the numbers in a region must equal the given number.