N = int(input())

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

ans = []
mod = 12
for D in range(13):

    flag = True

    for t in T:

        if t not in [D % mod,(D+2)%mod,(D+4)%mod,(D+5)%mod,(D+7)%mod,(D+9)%mod,(D+11)%mod]:
            flag = False
            break

    if flag:
        ans.append(D)

if len(ans) == 1:
    print (ans[0])
else:
    print (-1)