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

l_ck = [0, 2, 4, 5, 7, 9, 11]
ans = []
for i in range(12):
    f = 0
    for l_t_i in l_t:
        ck = l_t_i - i 
        if ck >11:
            ck -= 12
        if ck < 0:
            ck += 12
        if ck in l_ck:
            pass
        else:
            f = 1
            break
    if f == 0:
        ans.append(i)

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