aim of Kakuro is to fill in the blank squares using the numerals 1 through 9. A circled number, a.k.a. a “clue,” tells the sum of the numbers that should go in the adjacent open squares. A number in the top right corner is an “across” clue; and one in the bottom left, a “down” clue.
No number appears twice in any solution to a clue. For example, if the clue is 6 and you have two open squares, you could have 1 and 5, or 2 and 4, but not 3 and 3.