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Plays with polygon

Source: St Petersburg Olympiad 2011, Grade 9, P7

September 15, 2017
combinatoricsnumber theory

Problem Statement

Sasha and Serg plays next game with 100100-angled regular polygon . In the beggining Sasha set natural numbers in every angle. Then they make turn by turn, first turn is made by Serg. Serg turn is to take two opposite angles and add 11 to its numbers. Sasha turn is to take two neigbour angles and add 11 to its numbers. Serg want to maximize amount of odd numbers. What maximal number of odd numbers can he get no matter how Sasha plays?