a_{n+2}=a_{n+1}-a_n

a_{n+3}=a_{n+2}-a_{n+1}

Addition of two expressions , we get

a_{n+3}=-a_n

Therefore

a_{n+6}=a_n

So the terms of this sequence has a period of 6 and repeat themselves every 6 terms.

a_{2017} = a_{336\times 6+1} =a_1 = 2