N=int(input())
T=set(list(map(int,input().split())))
x=[0,2,4,5,7,9,11]
ans=list()
for D in range(12):
    DL=set([(D+i)%12 for i in x])
    if DL>=T:
        ans.append(D)

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