how to play kakuro

A Kakuro game is a grid with free and blocked cells like the one above.

 In free cells, the user can enter the numbers 1 to 9.

 If a blocked cell contains a slash and small numbers then it is called a clue cell.
 The small number denotes the sum clue for the adjacent group of free cells. In this case the horizontally adjacent group has a sum of 13 and the vertically adjacent group has a sum of 4.

The object of the game is to fill all free cells with numbers 1 to 9 so, that:

a) Any free cell in a group has a unique number.
b) The sum of all free cells in a group equals the sum clue associated with it.