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.

the chinese postman problem is a mathematical problem of graph theory.
chinese postman problem

barbara boxer (born november 11, 1940) is an american politician.
barbara boxer

the wrigley building (400-410 north michigan avenue, near north side, chicago, illinois) is a skyscraper in chicago.

wrigley building