Sentence:  For example , the marksman gets 5 shots , but we take his score to be the number of shots before his first bull's-eye , that is , <WSD>0</WSD> , 1 , 2 , 3 , 4 ( or 5 , if he gets no bull's-eye ) . 
Meaning of the word marked with <WSD> tags:  a mathematical element that when added to another number yields the same number