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.

Ex Input:
causality is a way to describe how different events relate to one another.

Ex Output:
causality


Ex Input:
a mathematician's apology is a book about mathematics which was written in 1940.

Ex Output:
a mathematician's apology


Ex Input:
shiboprosad mukherjee (born 20 may 1974) is an indian bengali filmmaker, writer and actor.

Ex Output:

shiboprosad mukherjee