def f(D):
    return [D%12,(D+2)%12,(D+4)%12,(D+5)%12,(D+7)%12,(D+9)%12,(D+11)%12]

N=int(input())
T=[int(t) for t in input().split()]
cnt=0

s=set()
for i in range(12):
    if sum(t in f(i) for t in T)==len(T):
        cnt+=1
        s.add(i)
if len(s)==1:
    print(s.pop())
else:
    print(-1)