n = int(input())
a = list(map(int,input().split()))
res = []

for i in range(12):
    b = [(j + i) % 12 for j in (0, 2, 4, 5, 7, 9, 11)]
    ok = True
    for j in a:
        if j not in b:
            ok = False
            break
    if ok:
        res.append(i)

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