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

cnt = 0
ans = -1
for i in range(12):
    d = [i%12, (i+2)%12, (i+4)%12, (i+5)%12, (i+7)%12, (i+9)%12, (i+11)%12]
    f = True
    for t in T:
        if t not in d:
            f = False
    if f:
        cnt += 1
        ans = i

if cnt == 1:
    print(ans)
else:
    print(-1)