import bisect
import copy
import decimal
import fractions
import functools
import heapq
import itertools
import math
import random
import sys
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import degrees, gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines

N=int(readline())
A=list(map(int,readline().split()))
for i in range(N):
    if A[i]==-1:
        A[i]=0
for bit in range(30):
    for i in range(N-1,-1,-1):
        if i>>bit&1:
            A[i^1<<bit]+=A[i]
for i in range(N):
    A[i]**=2
for bit in range(30):
    for i in range(N):
        if i>>bit&1:
            A[i^1<<bit]-=A[i]
g=0
for a in A:
    g=GCD(g,a)
if g==0:
    g=-1
print(g)