A1324.[COCI-2007_2008-contest1]#4 SREDNJI

Consider a sequence A of integers, containing N integers between 1 and N. Each integer appears exactly once in the sequence.
A subsequence of A is a sequence obtained by removing some (possibly none) numbers from the beginning of A, and then from the end of A.
Calculate how many different subsequences of A of odd length have their median equal to B. The median of a sequence is the element in the middle of the sequence after it is sorted. For example, the median of the sequence {5, 1, 3} is
3.

The first line contains two integers, N (1 ≤ N ≤ 100000) and B (1 ≤ B ≤ N).
The second line contains N integers separated by spaces, the elements of sequence A.

Output the number of subsequences of A whose median is B.

In the fourth example, the four subsequences of A with median 4 are {4}, {7, 2, 4}, {5, 7, 2, 4, 3} and
{5, 7, 2, 4, 3, 1, 6}.