N = int(input())
tigau = [1, 3, 6, 8, 10]
T = list(map(int, input().split()))
cho = set()
for D in range(12):
    ok = True
    for t in tigau:
        if (D + t) % 12 in T:
            ok = False
            break
    if ok:
        cho.add(D)
if len(cho) == 1:
    for i in cho:
        print(i)
else:
    print(-1)