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: Adaptation traits contribute to individual fitness and to the evolution of animal genetic resources. 
Sentence2: Behaviors can improve the evolution of animal genetic resources.

Ex Output:
animal


Ex Input:
Sentence1: cell division often causes growth. 
Sentence2: Mitosis is cell division.

Ex Output:
cell


Ex Input:
Sentence1: a doorbell converts electrical energy into sound. 
Sentence2: a doorbell converts  kinetic  energy into sound.

Ex Output:

a