https://aops.com/community/p11396666 Supercali wrote: C1 can be overkilled by using Four Colour Theorem on the map+sea. Just note that if the sea is coloured suppose red, then red cannot be used to colour any region on land. Hence, we have used 3 colours. https://aops.com/community/p11412589 cooljoseph wrote: I'm not sure if this would get many points. Consider the sea to be another region. It is well known that every map is 4-colorable. Because every region touches the sea, no other region can have the same color as the sea. Thus, the regions of the country must be 3-colorable.