n = int(input())
tlst = list(map(int, input().split()))

dd = [0, 2, 4, 5, 7, 9, 11]
cand = []
for i in range(12):
    lst = []
    for d in dd:
        lst.append((i + d) % 12)
    for t in tlst:
        if t not in lst:
            break
    else:
        cand.append(i)
if len(cand) == 1:
    print(cand[0])
else:
    print(-1)