N, *T = map(int, open(0).read().split())

ans = []
T = set(T)
for d in range(12):
    cand = [d, d + 2, d + 4, d + 5, d + 7, d + 9, d + 11]
    cand = set(c % 12 for c in cand)
    if all(t in cand for t in T):
        ans.append(d)

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