Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

Input: Consider Input: the calculus of variations is a field of mathematical analysis.

Output: calculus of variations


Input: Consider Input: the gates of the forest is a book written by nobel peace prize-winning author elie wiesel.

Output: the gates of the forest


Input: Consider Input: ob stars are hot, massive blue stars.


Output: ob star