https://olymp.hse.ru/data/2019/12/17/1523224503/2020_%D0%BA%D1%80%D0%B8%D1%82%D0%B5%D1%80%D0%B8%D0%B8%20%D0%BE%D0%BF%D1%80%D0%B5%D0%B4%D0%B5%D0%BB%D0%B5%D0%BD%D0%B8%D1%8F%20%D0%B4%D0%B8%D0%BF%D0%BB%D0%BE%D0%BC%D0%B0%D0%BD%D1%82%D0%BE%D0%B2_1%20%D1%8D%D1%82%D0%B0%D0%BF.pdf