Many Roads

The mighty river ByteStream runs through the fertile lands of ByteLands. The 2N cities of ByteLand are split by the river perfectly, N cities on one side, and N cities on the other. On the north side, cities are labeled 1 through N from west to east. Similarly, on the south side of the river, cities are labeled N+1 through 2N from west to east. Let’s consider the following image:


This is a companion discussion topic for the original entry at https://toph.co/p/many-roads