http://acm.hdu.edu.cn/showproblem.php?pid=4902 (hdu4902)
http://acm.hdu.edu.cn/showproblem.php?pid=4893 (hdu4902)
http://codeforces.com/contest/447/problem/E (Codeforces Round #FF (Div. 2) E))
http://codeforces.com/contest/121/problem/E (Codeforces Beta Round #91 (Div. 1 Only))