N = int(input())
T = list(map(int,input().split()))
t = set(T)
mod = 12
a = (0,2,4,5,7,9,11)

ans = []
for d in range(12):
    s = set()
    for k in a:
        s.add((d + k) % mod)
    if t <= s:
        ans.append(d)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)