A map of towns, each at [x, y]. You may keep one straight road, and every town moves onto it, to its shadow (as in “The shadow”). Write main_direction(points): which way the road should run to keep the towns as spread out as possible, as an arrow [dx, dy] of length 1. Where the road sits doesn’t matter, only its direction, and either sign is fine: [1, 0] and [-1, 0] are the same road.