SP3273 ORDERSET - Order statistic set

[English](/problems/ORDERSET/en/) [Vietnamese](/problems/ORDERSET/vn/)In this problem, you have to maintain a dynamic set of numbers which support the two fundamental operations - INSERT(S,x): if x is not in S, insert x into S - DELETE(S,x): if x is in S, delete x from S and the two type of queries - K-TH(S) : return the k-th smallest element of S - COUNT(S,x): return the number of elements of S smaller than x

- Line 1: Q (1 ≤ Q ≤ 200000), the number of operations - In the next Q lines, the first token of each line is a character I, D, K or C meaning that the corresponding operation is INSERT, DELETE, K-TH or COUNT, respectively, following by a whitespace and an integer which is the parameter for that operation. If the parameter is a value x, it is guaranteed that 0 ≤ |x| ≤ 10 $ ^{9} $ . If the parameter is an index k, it is guaranteed that 1 ≤ k ≤ 10 $ ^{9} $ .

For each query, print the corresponding result in a single line. In particular, for the queries K-TH, if k is larger than the number of elements in S, print the word 'invalid'.