n = int(input())
t = list(map(int,input().split()))

base = set([0,2,4,5,7,9,11])
ans = -1
for i in range(12):
    for j in t:
        if(not (j-i)%12 in base):
            break
    else:
        if(ans==-1):
            ans = i
        else:
            print(-1)
            exit()

print(ans)