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