Fans of math puzzle games will enjoy Kakuro, a challenging Flash game that's a combination of Sudoku and crosswords. The objective of the game is to provide a group of numbers from 1-9 that add up to the target sums on the blue squares. Similar to Sudoku, each number can only be used once for a target sum. If a target sum appears in the upper right hand corner of the blue square, this means that the numbers you use will go to the right of it (Across). If a target sum appears in the lower left hand corner, this means that the numbers you use will go below it (Down).

The challenges lie in determining the right combinations and orders of numbers to get that sum. For example, if an Across target sum is 7 and there are two spaces next to it, you can use 1+6, 2+5, or 3+4. However, you can also use 6+1, 5+2, and 4+3. And just like a crossword puzzle, the numbers you use for the Across target sum has to coincide with the Down target sum. There is only one solution per board.

Kakuro offers three difficulty levels: Easy, Medium, and Hard, with randomized boards for each level. There's also a Practice level available. The board itself is very user-friendly. Clicking on the target sums brings up a list of the possible number combinations. The numbers are color-coded; green means you have the correct sum while red means you have the wrong sum. A yellow number indicates that the sum is wrong in one direction. You can type in the numbers directly or select a number from the drop-down keypad. If you're not sure of a certain number, you can mark it and return to it later. If you put the wrong number, you can always delete it or hit the Undo icon. And if you really screw up, you can clear the board and start over.

I've noticed that it helps to solve the small sums first, then continue to the larger sums that require a long string of number combinations. In that sense, it's very much like a crossword puzzle. Still, there's a bit of trial and error involved as you play around with various orders and combinations of numbers that will answer both the Across and Down target sums.

Kakuro is a refreshing twist on the classic crossword puzzle and the highly popular Sudoku. Hardcore puzzle fans may feel that the availability of number combinations list makes the game too easy, but all they have to do is not click on the target sums. Besides, if you really want to challenge yourself, you can always do it the old-fashioned way. Print out the Hard Level board and complete it...with a pen!