n=int(input())
t=list(map(int,input().split()))
a=[]
for i in range(12):
  f=1
  f&=i in t
  f&=all((i+j)%12 not in t for j in [1,3,6,8,10])
  if f:
    a+=[i]
print(a[0] if len(a)==1 else -1)