n = int(input())
t = list(map(int, input().split()))
cnt = 0
for d in range(12):
    ok = True
    for ti in t:
        flg = False
        for k in [0, 2, 4, 5, 7, 9, 11]:
            if ti == (d + k) % 12:
                flg = True
        if not flg:
            ok = False
    if ok:
        ans = d
        cnt += 1
if cnt == 1:
    print(ans)
else:
    print(-1)