Here's a short story:

Korea is a historic country in East Asia, since 1945 divided into two distinct sovereign states: North Korea (officially the "Democratic People's Republic of Korea") and South Korea (officially the "Republic of Korea"). Located on the Korean Peninsula, Korea is bordered by China to the northwest and Russia to the northeast. It is separated from Japan to the east by the Korea Strait and the Sea of Japan (East Sea). 

Korea emerged as a singular political entity after centuries of conflict among the Three Kingdoms of Korea, which were unified as Later Silla to the south and Balhae to the north. Later Silla divided into three separate states during the Later Three Kingdoms period. Goryeo, which had succeeded Goguryeo, defeated the two other states and united the Korean Peninsula. Around the same time, Balhae collapsed and its last crown prince fled south to Goryeo. Goryeo (also spelled as "Koryŏ"), whose name developed into the modern exonym "Korea", was a highly cultured state that created the world's first metal movable type in 1234. However, multiple invasions by the Mongol Yuan Dynasty during the 13th century greatly weakened the nation, which eventually agreed to become a vassal state after decades of fighting. Following the Yuan Dynasty's collapse, severe political strife followed, and Goryeo eventually fell to a coup led by General Yi Seong-gye, who established Joseon in 1392. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: When did Korea become one country?
Guess: after centuries of conflict

Question: During these conflicts, what was the southern kingdom called?
Guess: Silla

Question: And the northern kingdom?
Guess: Balhae

Question: Which kindom does Korea take its name from?
Guess: Goryeo

Question: What did it invent?
Guess: metal movable type

Question: Whom did it become subordinate to?
Guess: Mongol Yuan Dynasty

Question: When?
Guess: 13th century

Question: What succeeded Goryeo?
Guess: Joseon

Question: Who caused that?
Guess: Yi Seong-gye

Question: What gave him the opportunity to do that?
Guess: severe political strife

Question: Caused by?
Guess: Yuan Dynasty's collapse

Question: When did Korea split again?
Guess: since 1945

Question: Which is referred to as the DPRK?
Guess: North Korea

Question: How many countries does Korea share a land border with?
Guess: Two

Question: What are the names of the two countries?
Guess: China and Russia

Now answer the following question in the same format.

Question: What country does Korea share a sea border with?

Japan