Each dot represents a city with population > 1 million. To turn the dot green, guess any 1M city within 1,000 kilometers.
UrbanArea population according to citypopulation.de (ref. date 1.1.19)
Dot size depends on population
Dot colours: blue represents a capital, dark blue an agglomeration including a capital, red represents an agglomeration without a main city, white on the circumference represents a city with no other 1M city within 1,000 km