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

Example Input: venice is a city in italy.
Example Output: venice

Example Input: thallium(i) iodide, also known as thallous iodide, is a chemical compound.
Example Output:
thallium(i) iodide