Normal sudoku rules apply. (ie fill the grid with the numbers from 1 to 9 so that in each row, column, and 3x3 block, each digit occurs exactly once). For every triple of adjacent digits on a line, one of the digits has to be the sum of the other two, e.g. 2-5-7, 6-9-3, and 8-4-4 are valid adjacent digits along a line. Digits may repeat on a line if allowed by the other rules. NB Each line in the shape of a Y consists of three lines, each of three cells length. For instance, the line running in boxes 1 and 4 contains the three lines R2C2-R3C2-R4C1, R2C2-R3C2-R4C3, and R4C1-R3C2-R4C3.