MOD = 12

def func(a):
    ret = set()
    for i in [0, 2, 4, 5, 7, 9, 11]:
        ret.add((a + i) % 12)
    return ret

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

ans = -1

for i in range(12):
#     print(func(i))
#     if i == 2:
#         print("T <= func(i) : ", T <= func(i))
    if T <= func(i):
        if ans == -1:
            ans = i
        else:
#             print("ans =", ans)
            ans = -1
#             print("func(i) =", func(i))
            break
print(ans)