Number Pairs

You are given a sequence of $N$ distinct integers. Count how many pairs of elements in the sequence add up to $K$.

A pair is made of two elements at different positions, and the order inside a pair does not matter, so $(1, 6)$ and $(6, 1)$ are the same pair. The same element cannot be used twice, so when $K$ is even and $K/2$ appears in the sequence, that single value does not form a pair by itself.

For the sequence $1, 2, 3, 4, 5, 6$ and $K = 7$ the pairs $(1, 6)$, $(2, 5)$, $(3, 4)$ satisfy the condition, so the answer is $3$. For the same sequence and $K = 5$ the pairs $(1, 4)$ and $(2, 3)$ satisfy it, so the answer is $2$.

The first line contains the sequence length $N$ and the target sum $K$, separated by a space.

The second line contains the $N$ elements of the sequence, separated by spaces.

$1 \le N \le 1\,000$. The elements are pairwise distinct integers between $1$ and $10^6$. $K$ is an integer between $1$ and $2 \times 10^6$.

Print one integer, the number of pairs whose sum is $K$.