Question: The confucian academy, Dosan Seowan, located in the East Asian country bordered by China to the northwest and Russia to the northeast, was founded by two prominant Confucian scholars, one being known as Yi Yulgok. What was the "pen" name of the other founding scholar?
Sentence (from "Yi Hwang"): Yi Hwang is often referred to by his pen name Toegye ("Retreating Creek").
Relevant evidence: Yes