Given an array arr, find element pairs whose sum equal the second argument arg and return the sum of their indices.

pairwise.py

def pairwise(arr, arg):
    size = range(len(arr))
    s = 0
    for i in size:
        for j in size:
            if arr[i] + arr[j] == arg:
                arr[i] = 0
                arr[j] = 0
                s = s + i + j
    return s