N = int(input())
T = [int(t) for t in input().split()]

ans = -1
for i in range(12):
    L = [i, i+2, i+4, i+5, i+7, i+9, i+11]
    for j in range(len(L)):
        L[j] %= 12
    flag = True
    for t in T:
        if not t in L:
            flag = False
            break
    if flag:
        if ans > -1:
            ans = -1
            break
        ans = i

print(ans)