Read this: Charleston has one official sister city, Spoleto, Umbria, Italy. The relationship between the two cities began when Pulitzer Prize-winning Italian composer Gian Carlo Menotti selected Charleston as the city to host the American version of Spoleto's annual Festival of Two Worlds. "Looking for a city that would provide the charm of Spoleto, as well as its wealth of theaters, churches, and other performance spaces, they selected Charleston, South Carolina, as the ideal location. The historic city provided a perfect fit: intimate enough that the Festival would captivate the entire city, yet cosmopolitan enough to provide an enthusiastic audience and robust infrastructure."

What isn't the name of festival held in Spoleto?
What is the answer? (If it cannot be answered, return "unanswerable")
unanswerable