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:
a releaser is a stimulus from one animal to another, which causes a particular response.

Ex Output:
releaser


Ex Input:
the canton of bellerive-sur-allier is an administrative division of the allier department in central france.

Ex Output:
canton of bellerive-sur-allier


Ex Input:
iron(ii) hydroxide is a chemical compound.

Ex Output:

iron(ii) hydroxide