a = [0, 2, 4, 5, 7, 9, 11]
n = int(input())
t = [int(v) for v in input().split()]
ans = []
for i in range(12):
    b = [(i + v) % 12 for v in a]
    print(i, b)
    c = 0
    for j in t:
        if j in b:
            c += 1
    if c == n:
        ans.append(i)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)