def is_tone_notes(root: int) -> set[int]:
    return {(root + interval) % 12 for interval in (0, 2, 4, 5, 7, 9, 11)}


def main():
    N = int(input())
    T = set(map(int, input().split()))

    tones = set()
    for root in range(12):
        if is_tone_notes(root).issuperset(T):
            tones.add(root)

    if len(tones) == 1:
        print(tones.pop())
    else:
        print(-1)


if __name__ == "__main__":
    main()