Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Ex Input:
Sentence1: Muscles can only contract. 
Sentence2: Contraction is a type of movement.

Ex Output:
contract


Ex Input:
Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: Antigens are proteins foreign to the body.

Ex Output:
antigen


Ex Input:
Sentence1: Population size influences the chances of a species surviving. 
Sentence2: population size influences the chances of flowers surviving.

Ex Output:

Population