In the following paper (Example 2.1), it has been mentioned to K+M to provide an example of a pseudo valuation domain which is not a valuation ring, and its reference is Gilmer's book, but I have no access to Gilmer's book.

Can someone help me and explain what K+M is?

"Hedstrom, J.R., Houston, E.G.: Pseudo-valuation domains, Pacific J. Math., 4 (1978) , 199–207"