Fill in the missing numbers with the digits 1 to 9. A diamond shape in the middle means that the four numbers around it add to 20.
Each uses a different way to add to 20, i.e. if there is already 1 + 3 + 7 + 9, then there will not be another using the digits 1, 3, 7 and 9 (in any order).
The same digit isn't allowed to touch, even diagonally.