Normal sudoku rules apply. Shade the grid such that all shaded cells are orthogonally connected, all unshaded cells are orthogonally connected, and no 2x2 area is entirely shaded or unshaded. Circles are unshaded, and count the number of shaded cells in the (up to 8) neighbouring cells. Digits along a green line sum to a concatenation of the two digits in the connected circles. For example 3_436_1 would be a valid fill as 436 sums to 13 which can be made from the connected 3 and 1.