Task: Question Generation
Input (English): Passage: In 1571 the Crimean Khan burned Moscow. He tried again the next year and was defeated. Another large raid failed in 1591. The raiding area may have moved slightly south and east. Attacks east of Ryazan are mentioned more frequently. After 1585 forts were built far to the south to block the Muravsky Trail, but these had no visible effect in this period. Raids on Poland are mentioned under 1577, 1584 and 1595.
Output (English): How many different years were raids on Poland mentioned?