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:
in cryptography, a pre-shared key or psk is a shared secret which was previously shared between the two parties using some secure channel before it is used.

Ex Output:
pre-shared key


Ex Input:
spintronics is a kind of electronics that uses the tiny magnetic fields that come from electrons as well as their electric charge.

Ex Output:
spintronics


Ex Input:
"morning in america" is the common name of a political campaign television commercial.

Ex Output:

morning in america