def solve(N, A):
   if all(a == 0 for a in A):
      return 1<<int.bit_length(N)

   def resolve(e):
      I = [(i>>e)&1 for i in range(1, N+1)]
      B = [(a>>e)&1 for a in A]
      ng = [0, 0]
      for i, b in zip(I, B):
         if i == 0 and b == 1:
            ng[0] = ng[1] = 1
         if i == 1 and b == 0:
            ng[1] = 1
         if i == 1 and b == 1:
            ng[0] = 1
      if not ng[0]:
         return 0
      if not ng[1]:
         return 1
      return -1

   maxbitlen = int.bit_length(max(N, max(A))) + 2
   res = 0
   for e in range(maxbitlen):
      x = resolve(e)
      if x == -1:
         return -1
      res |= x<<e
   return res


N = int(input())
A = list(map(int, input().split()))
ans = solve(N, A)
print(ans)