Distinct Pairs of Stocks

Count the unordered value pairs that can be formed from two distinct indices and sum to target. The same value pair counts only once; (a,a) requires at least two a.

输入格式

The first line contains n target (1≤n≤500000, 0≤target≤5×10⁹); the second line contains n profits (0..10⁹).

输出格式

Output a single integer answer.

样例 1

输入：

7 12
6 6 3 9 3 5 1

输出：

2

样例 2

输入：

1 12
6

输出：

0

样例 3

输入：

3 0
0 0 0

输出：

1

样例说明

Example 1: only (3,9) and (6,6), so the answer is 2. Example 2: a single 6 cannot pair with itself, so the answer is 0. Example 3: three 0s give three index pairs, but the only value pair is (0,0), so the answer is 1.