n=int(input())
t=[*map(int,input().split())]
chk=[0,2,4,5,7,9,11]
result=-1
for i in range(12):
    check=set((i+j)%12 for j in chk)
    for x in t:
        if x not in check:
            break
    else:
        if result==-1:
            result=i
        else:
            print(-1)
            exit()
print(result)