n = int(input())
t = list(map(int, input().split()))
nums = [0, 2, 4, 5, 7, 9, 11]
mod = 12

ans = -2
for d in range(12):
    cnd = [(d + e) % mod for e in nums]
    bl = True
    for e in t:
        if e not in cnd:
            bl = False

    if bl:
        if ans > -2:
            ans = -1

        else:
            ans = d

if ans == -2:
    ans = -1

print(ans)