l = [0, 2, 4, 5, 7, 9, 11]
n = int(input())
t = list(map(int, input().split()))
ans = []
for i in range(12):
    for j in range(n):
        if (t[j] - i) % 12 not in l:
            break
    else:
        ans.append(i)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)