Find Pair Sum Equal to K

Given an integer array arr and an integer 'Sum', find and return the total number of pairs in the array which, when added together, result in the 'Sum'.

Note:

The array can contain duplicate elements. Pairs (arr[i], arr[j]) and (arr[j], arr[i]) are considered the same.

Input:

First line: two space-separated integers, N and Sum. Second line: N space-separated integers representing array elements.

Output:

Total number of pairs present in the array that sum to the given value.

Example:

Input:
N = 5, Sum = 5
arr = [1, 2, 3, 2, 1]
Output:
2
Explanation:
The pairs (1, 4) and (2, 3) sum to 5.

Constraints:

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