N,*T=map(int,open(0).read().split())
x=[0,2,4,5,7,9,11]
a=-1
for i in range(12):
 t=set(i+e for e in x)
 for e in T:
  if e not in t:
   break
  else:
   if a==-1:
    a=i
   else:
    print(-1)
    exit()
print(a)