Original writeup (http://magic-hat.ru/for400phdays-quals-2012-%D1%81%D0%BA%D0%B0%D0%B7-%D0%BE-%D1%82%D0%BE%D0%BC-%D0%BA%D0%B0%D0%BA-%D0%B0%D0%B4%D0%BC%D0%B8%D0%BD-%D0%B8-%D1%80%D0%B5%D0%B2%D0%B5%D1%80%D1%81%D0%B5%D1%80).