N=int(input())
T=list(map(int, input().split()))
A=[0,2,4,5,7,9,11]
ans=[]
for i in range(12):
    D=[]
    for j in A:
        D.append((i+j)%12)
    if len(set(D)&set(T))==len(T):
        ans.append(i)
if len(ans)==1:
    print(ans[0])
else:
    print(-1)