After Duke Otto III of Pomerania-Stettin had died of the plague in 1464, Frederick used his claim use suzerainty of Brandenburg over Pomerania, an issue which had never been clarified, to lay claim on Pomerania-Stettin. On 21 January 1466, the Dukes and the Elector concluded the Treaty of Soldin, in which the Elector enfeoffed the Dukes with Pomerania and they recognised him as their liege lord. The Pomeranian Dukes, however, did not meet their treaty obligations and the conflict erupted again. In 1468, Brandenburg captured several Pomeranian towns on both sides of the Oder. After an unsuccessful siege of Ueckermünde, the two sides agreed to a cease-fire. Peace negotiations in Piotrków Kujawski only resulted in an extension of the cease-fire. In May 1470, Eric II invaded the Neumark and started looting. In the meantime, Emperor Frederick III acknowledged the claims of Brandenburg. The emperor enfeoffed Frederick II with Pomerania-Stettin and order Eric II and Wartislaw X to recognise Frederick as their liege lord. Duke Henry IV of Mecklenburg mediation, and at the end of May 1472, a lasting peace treaty was signed at Prenzlau. The Dukes and the Estates of Pomerania had to pay homage to the Frederick II, who was also allowed to keep the territories he had conquered. The Dukes retained a number of law professors from the University of Greifswald to review the peace treaty. Among them were Johannes Parleberg, Matthias von Wedel, Sabel Siegfried the Younger, Hermann Slupwachter, Johann Elzing, Heinrich Zankenstede and Georg Walter. Gerwin Rönnegarwe and Hertnidt vom Stein also played a role.
Question: How many law professors from the University of Greifswald did the Duke retain to review the peace treaty?
Answer: 9