#yuki1109
n=int(input())
t=list(map(int,input().split()))
r=[0,2,4,5,7,9,11]
res=[]
for i in range(12):
 for j in range(n):
  if (t[j]-i)%12 not in r:
   break
 else:
  res.append(i)
if len(res)==1:
 print(res[0])
else:
 print(-1)