x = [0, 2, 4, 5, 7, 9, 11]

ans = []
N = int(input())

T = list(map(int, input().split()))
for i in range(12):
    if all((t - i) % 12 in x for t in T):
        ans += [i]
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)