import sys
input = lambda: sys.stdin.readline().strip()

_ = input() # N
T = set(map(int, input().split()))

def C(d):
    return set(map(lambda x: x % 12, [d, d+2, d+4, d+5, d+7, d+9, d+11]))

ans = [D for D in range(12) if T <= C(D)]

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