已知 a>0, S1=1a, S2=−S1−1, S3=1S2, S4=−S3−1, S5=1S4, …(即当 n为大于1的奇数时, Sn=1Sn−1;当 n为大于1的偶数时, Sn=−Sn−1−1),按此规律, S2018= .