There are 6 problems whose rating >=4500 left::
http://acm.timus.ru/problem.aspx?space=1&num=1999
http://acm.timus.ru/problem.aspx?space=1&num=1674
http://acm.timus.ru/problem.aspx?space=1&num=1596
http://acm.timus.ru/problem.aspx?space=1&num=1388
http://acm.timus.ru/problem.aspx?space=1&num=1589
http://acm.timus.ru/problem.aspx?space=1&num=1394
all of them seems not easy to solve, welcome to discuss these problems..