n = int(input())
T = list(map(int, input().split()))

ANS = []
for d in range(12):
    seen = [False for _ in range(n)]
    for t in [0, 2, 4, 5, 7, 9, 11]:
        c = (d + t) % 12
        for i in range(n):
            if T[i] == c:
                seen[i] = True
                break
    flag = True
    for i in range(n):
        if not seen[i]:
            flag = False
            break
    if flag:
        ANS.append(d)
if len(ANS) == 1:
    print(ANS[0])
else:
    print(-1)