Count Pairs with Given Sum

Given an integer array/list arr and an integer 'Sum', determine the total number of unique pairs in the array whose elements sum up to the given 'Sum'.

Input:

The first line contains two space-separated integers, N and Sum.
The second line contains N space-separated integers representing the elements of the array.

Output:

Output a single integer, representing the total number of pairs whose sum is equal to 'Sum'.

Example:

Input:
5 7
1 2 3 4 3
Output:
2

Constraints:

• 1 ≤ N ≤ 105
• -104 ≤ Sum ≤ 104
• -104 ≤ arr[i] ≤ 104
• Time Limit: 1 sec

Note:

Array may contain duplicate elements.
The pairs (arr[i], arr[j]) and (arr[j], arr[i]) are considered the same.