def sales_by_match(n, ar):
    #Time complexity: O(n)
    #Space complexity (ignoring input): O(n)
    socks_dict = {}
    for sock in ar:
        if sock in socks_dict:
            socks_dict[sock] += 1
        else:
            socks_dict[sock] = 1

    total_pairs = 0
    for values in socks_dict.values():
        total_pairs += values // 2

    return total_pairs