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1

Part of 2021 Indonesia MO

Problems(1)

Add (+) or (-)?

Source: Indonesia National Math Olympiad 2021 Problem 1 (INAMO 2021/1)

11/8/2021
On the whiteboard, the numbers are written sequentially: 1 2 3 4 5 6 7 8 91 \ 2 \ 3 \ 4 \ 5 \ 6 \ 7 \ 8 \ 9. Andi has to paste a ++ (plus) sign or −- (minus) sign in between every two successive numbers, and compute the value. Determine the least odd positive integer that Andi can't get from this process.
combinatorics