def ip():return int(input())
def mp():return map(int, input().split())
def lmp():return list(map(int, input().split()))
# No.3249 AND
N = ip()
B = lmp()
if N == 1 and B[0] == 0:
  print(2)
  exit()
x = 0
for i, b in enumerate(B, 1):
  x |= i & b
for i, b in enumerate(B, 1):
  if b != i & x:
    print(-1)
    exit()
print(x)