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

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

res = -1

for i in range(12):
    for j in range(N):
        if not T[j] in [(i + d[k]) % 12 for k in range(7)]:
            break
    else:
        if res == -1:
            res = i
        else:
            print(-1)
            break
else:
    print(res)