Return a unique integer from a set of two lists

unique_int_from_two_lists.py

def solution(x, y):
    unique = None
    work_list = longest([x, y])
    if work_list == x:
        compare_list = y
    else:
        compare_list = x

    for i in work_list:
        if i not in compare_list:
            unique = i

    return unique


def longest(paths):
    return max(paths, key=len)


solution([14, 27, 1, 4, 2, 50, 3, 1], [2, 4, -4, 3, 1, 1, 14, 27, 50])