Minimize reduction cost and validate equal-sum pairs

You are given two independent coding tasks.

Task 1: Minimum total cost to reduce an array

You are given an integer array arr of length n.

You may repeatedly perform the following reduction operation until only one number remains:

1. Choose any two elements x and y currently in the array.
2. Remove both from the array.
3. Insert their sum x + y back into the array.
4. The cost of this operation is x + y .

Return the minimum possible total cost over all sequences of operations.

Input: arr (integers)

Output: minimum total cost (use 64-bit integer)

Notes/assumptions (make explicit in your solution):

• n >= 1 .
• Elements can be any integers; total cost should be computed using 64-bit arithmetic.

Task 2: Pair elements into equal-sum groups and sum products

You are given an integer array arr of even length 2m.

You must partition the array into m disjoint pairs such that:

• Every element is used in exactly one pair.
• The sum of the two numbers in each pair is the same constant S for all pairs.

If such a partition is possible, return:

• The value $\sum_{i=1}^{m} (a_i \times b_i)$ , i.e., the sum of products of each pair.

If it is not possible to form such pairs, return -1.

Input: arr (even length)

Output: sum of pairwise products, or -1 (use 64-bit integer)