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:
a video projector takes a video signal and projects the corresponding image on a screen using a lens system.

Ex Output:
video projector


Ex Input:
in greek mythology, the nereids ( "nereides") are sea nymphs, the fifty daughters of nereus, the old man of the sea, and the okeanid doris.

Ex Output:
nereid


Ex Input:
kot sarang is a village and union council, an administrative subdivision, of chakwal district in the punjab province of pakistan, it is part of talagang tehsil and is located at 33°2'14n 72°23'11e.

Ex Output:

kot sarang