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.

Example Input: the saint petersburg stadium (russian: Стадион Санкт-Петербург; also called the krestovsky stadium or zenit arena) is an association football stadium in saint petersburg, russia.
Example Output: saint petersburg stadium

Example Input: varicose veins are a condition in which the veins in the leg area have become enlarged.
Example Output: varicose veins

Example Input: one of guerrero's sons, vladimir guerrero jr., plays in mlb.
Example Output:
vladimir guerrero