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

n=int(input())

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

flag_ans = False
for i in range(12):
    flag_ok = True
    for j in t:
        if (i+j)%12 not in check:
            flag_ok = False
            break
    if flag_ok:
        if flag_ans:
            print(-1)
            exit()
        else:
            flag_ans = True
            ans = i

if flag_ans:
    print(ans)
else:
    print(-1)