赚现金
已知 a > 0 , S 1 = 1 a , S 2 = − S 1 − 1 , S 3 = 1 S 2 , S 4 = − S 3 − 1 , S 5 = 1 S 4 , … (即当 n 为大于1的奇数时, S n = 1 S n − 1 ;当 n 为大于1的偶数时, S n = − S n − 1 − 1 ) ,按此规律, S 2018 = .
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