n = int(input())
t = list(map(int, input().split()))
ans = []
for i in range(12):
    if all([(ti + 12 - i) % 12 in [0, 2, 4, 5, 7, 9, 11] for ti in t]):
        ans.append(i)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)