N = int(input())
t = list(map(int, input().split()))
ans = []

for d in range(12):
    # flag = False
    if all(sound in [d, (d+2) % 12, (d+4)%12, (d+5)%12, (d+7)%12, (d+9)%12, (d+11)%12] for sound in t):
        ans.append(d)

if len(ans) == 1:
    print(ans[0])
else:
    print(-1)