All or Nothing (474)

Draw a single closed loop that travels horizontally and vertically through the centres of some white cells. The loop cannot touch or cross itself. Every region must be visited by the loop at most once. When the loop enters a region, it must pass through all the cells of the region before leaving the region. No two unvisited cells belonging to distinct regions can touch each other orthogonally.

Reference: WPC 2016 (65)