n=int(input())
t=list(map(int,input().split()))
d=[0,2,4,5,7,9,11]
a=[]
for i in range(12):
  ok=True
  for tt in t:
    tmp=tt-i
    tmp%=12
    if tmp in d:
      continue
    else:
      ok=False
      break
  if ok:
    a.append(i)
if len(a)==1:
  print(a[0])
else:
  print(-1)