Task: Question Generation
Input (English): Passage: In November the Polish warrior-king Sobieski launched a major campaign on the right bank. Bar, Bratslav and Nemiroff surrendered to the Poles but Rashkov was taken by storm and the garrison slaughtered. Kalnik gave up and took the oath of allegiance to the Polish crown. Polish banners  under Dmitry Vishnevetsky and Stanislav Yablonovsky crushed Cossack and Tartar troops at Zhornishch and Nemiroff, and Polish-Lithuanian troops under Nicholas Senyavskaya and Prince Michael Casimir Radziwill defeated the enemy in the battles at Chyhyryn and Pavoloch. At the Battle of Pavoloch 4,000 Tatars under the Nureddin-Sultan was broken. The Nureddin-Sultan was killed. In autumn 1674 the Sultan organized a campaign against the Zaporozhian Sich, and sent 15,000 select Janissaries came by sea to Crimea. The Turkish government had decided to seize the Sich and destroy all the Zaporozhian Cossacks. By order of the Sultan, Selim Giray and 40,000 Tatars joined the Turkish campaign against Sich. In January 1675 15,000 Janissaries and 40,000 Tatars under Khan Selim Giray approached the Sich without being detected. Tartars surrounded the Sich and Janissaries tried to enter it, but the Zaporozhians repulsed the attack and decisively defeated the Turkish troops. 13,500 Janissaries were killed and the rest fled. The Khan retreated from the Sich back to the steppe.
Output (English): How many Janissaries weren't killed in the attack on the Sich?