Question

In a sequence of integers, A(n)=A(n-1)-A(n-2), where A(n) is the nth term in the sequence, n is an integer and n>=3, A(1)=1,A(2)=1. Calculate S(1000), where S(1000) is the sum of the first 1000 terms.
a..2 b.. 3 c..4 d..0