# [QBIC](https://drive.google.com/drive/folders/0B0R1Jgqp8Gg4cVJCZkRkdEthZDQ), 27 bytes

    g=(:+1)'\2`~(a-g)%2|g=-g}?g

## Explanation

    g=			set worker var 'g' to
    (:+1)   		our index (plus one for the ceil() bit)
    '\2`			integer divided by 2 (the int div needs a code literal: '..`
    ~(a-g)%2	IF index - temp result is odd (index 2 minus result 1 = 1)
    |g=-g		THEN negate g
    }			END IF
    ?g			print the result