Killer Sudoku

In this game there will be a grid of 9 x 9 blocks, this grid is further divided into 9 subgroups of 3 x 3 blocks, in addition to that, the blocks will be grouped by red clue lines marked with a number. The object of this game is to fill in all the blocks with 1 - 9 without repeat within either the rows or columns or the subgroups or the clue groups. Moreover, the numbers in a clue group must add up to the number given in the clue. This game is similar to sudoku but more challenging.