A sequence has terms a_{1}, a_{2}, a_{3}, \ldots. The first term is a_{1}=x and the third term is a_{3}=y. The terms of the sequence have the property that every term after the first term is equal to 1 less than the sum of the terms immediately before and after it. That is, when n \geq 1, a_{n+1}=a_{n}+a_{n+2}-1. The sum of the first 2018 terms in the sequence is
Answer Choices
A. -x-2 y+2023
B. 3 x-2 y+2017
C. y
D. x+y-1
E. 2 x+y+2015