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Albania BMO TST Problem 1

Source:

April 1, 2017
combinatorics

Problem Statement

Given nn numbers different from 00, (n∈Nn \in \mathbb{N}) which are arranged randomly. We do the following operation: Choose some consecutive numbers in the given order and change their sign (i.e. x→−xx \rightarrow -x). What is the minimum number of operations needed, in order to make all the numbers positive for any given initial configuration of the nn numbers?
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