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 beethoven house (german: beethoven-haus) in bonn, germany, is a memorial site, museum and cultural institution serving various purposes.

Ex Output:
beethoven house


Ex Input:
plankton are drifting organisms that live in the surface layers of the ocean.

Ex Output:
plankton


Ex Input:
"counter-strike: source" is essentially the same game on the source engine.

Ex Output:

counter-strike: source