n=int(input())
a=list(range(1,n+1))
b=list(map(int,input().split()))
c=0
for j in range(20):
  if all((b[i]>>j)&1==0 for i in range(n)):
    continue
  if any((a[i]>>j)&1!=(b[i]>>j)&1 for i in range(n)):
    print(-1)
    exit()
  c+=1<<j
if c>0:
  print(c)
else:
  print(1<<len(bin(a[-1])[2:]))