N = int(input())
T = set(map(int, input().split()))
ans = -1
for D in range(12):
    X = {D % 12, (D + 2) % 12, (D + 4) % 12, (D + 5) % 12, (D + 7) % 12, (D + 9) % 12, (D + 11) % 12}
    if T <= X:
        if ans != -1:
            ans = -1
            break
        else:
            ans = D
print(ans)