def solution(a):
    the_set = set()
    for element in a:
        if element > 0:
            the_set.add(element)

    if len(the_set) < 1:
        return 1

    for element in range(0, len(the_set)):
        if element + 1 not in the_set:
            return element + 1

    return len(the_set) + 1


assert solution([1, 3, 6, 4, 1, 2]) == 5

assert solution([1, 2, 3]) == 4

assert solution([-1, -3]) == 1

print("Ran Successfully")