CF1051C Vasya and Multisets

Vasya has a multiset $ s $ consisting of $ n $ integer numbers. Vasya calls some number $ x $ nice if it appears in the multiset exactly once. For example, multiset $ \{1, 1, 2, 3, 3, 3, 4\} $ contains nice numbers $ 2 $ and $ 4 $ . Vasya wants to split multiset $ s $ into two multisets $ a $ and $ b $ (one of which may be empty) in such a way that the quantity of nice numbers in multiset $ a $ would be the same as the quantity of nice numbers in multiset $ b $ .

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