Write a question about the following article: In 1426 the Hussites were again attacked by foreign enemies. In June 1426 Hussite forces, led by Prokop and Sigismund Korybut, significantly defeated the invaders in the Battle of Aussig. Despite this result, the death of Jan Žižka caused many, including Pope Martin V, to believe that the Hussites were much weakened. Martin proclaimed yet another crusade in 1427. He appointed Cardinal Henry Beaufort of England as Papal Legate of Germany, Hungary, and Bohemia, to lead the crusader forces. The crusaders were defeated at the Battle of Tachov. The Hussites subsequently invaded parts of Germany several times, though they made no attempt to occupy permanently any part of the country. Korybut was imprisoned in 1427 for allegedly conspiring to surrender the Hussite forces to Sigismund of Hungary. He was released in 1428, and participated in the Hussite invasion of Silesia. But after a few years, Korybut returned to Poland with his men. Korybut and his Poles, however, did not really want to leave; but the Pope threatened to call a crusade against Poland if they did not.
How many years was Korybut imprisoned?