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:
the british rail class 931 was reserved for former southern region electrical multiple units converted for departmental use.

Ex Output:
british rail class 931


Ex Input:
the wisconsin river is a river in the united states.

Ex Output:
wisconsin river


Ex Input:
in organic chemistry, an epoxide is a molecule that has a ring with at least three atoms in total, two carbon atoms and one oxygen atom.

Ex Output:

epoxide