Killer Sudoku

Just as in Sudoku, each row, column and 3x3 box must contain the numbers 1 to 9 only once. However, with Killer Sudoku, there's an extra challenge. Each group of squares joined by dotted lines must add up to the number in the top left hand box. Also, no number can be repeated within the dotted box. You'll soon understand how it got its name - it's a killer!