def solve():
    N = int(input())
    T = list(map(int, input().split()))
    cnt = 0
    ans = -1
    for D in range(12):
        lis = [D,D+2,D+4,D+5,D+7,D+9,D+11]
        for i in range(7):
            lis[i] %= 12
        for i in range(N):
            if T[i] not in lis:
                break
        else:
            if cnt==0:
                ans = D
                cnt += 1
            else:
                ans = -1
    return ans
print(solve())