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:
mariner 10 was a robotic nasa space probe.

Ex Output:
mariner 10

Ex Input:
colonel archibald gracie iv (january 17, 1859 december 4, 1912) was an american writer, amateur historian, real estate investor, and survivor of the sinking of the r.m.s titanic.

Ex Output:
archibald gracie iv

Ex Input:
electrocution or electric shock is when electricity passes through something living.

Ex Output:
electrocution