k=0
N=int(input())
R=list()
T=list(map(int,input().split()))
for i in range(12):
    s=0
    L=[i%12,(i+2)%12,(i+4)%12,(i+5)%12,(i+7)%12,(i+9)%12,(i+11)%12]
    for j in range(N):
        if T[j] in L:
            s+=1
    if s==N:
        k+=1
        R.append(i)
if k==1:
    print(R[0])
else:
    print(-1)