MathNet 1998 — Part 01 Question 36

The general term of a sequence $\{a_n\}$ is given by $a_{n+2} = a_{n+1} - a_n$. If the sum of the first 1997 terms is 1879 and the sum of the first 1879 terms is 1997, what is the sum of the first 2000 terms?