Normal sudoku rules apply. Digits in the quad circles must appear in the surrounding four cells. Digits in some quads must be deduced. The white segments represent 'Difference Fences'. Each segment has a value equal to the difference between the cells they separate. On a fence, no two segments can have the same value. Each fence has a total value equal to the sum of all the segments on the fence and this total value is given in the attached quad (it may be a 2-digit number). Some total values will need to be deduced.