n = int(input())
T = list(map(int, input().split()))

d = [0, 2, 4, 5, 7, 9, 11]

ans = -1

for i in range(12):
    t = set((i + j) % 12 for j in d)
    for j in T:
        if j not in t:
            break
    else:
        if ans == -1:
            ans = i
        else:
            print(-1)
            exit()
print(ans)