n = int(input())
B = list(map(int,input().split()))
ans = [0 for _ in range(20)]



for i in range(n):
    b = B[i]
    for j in range(20):
        if ((i+1) >> j) & 1:
            if (b >> j) & 1:
                ans[j] = 1
                
if not(any(ans)):
	print(-1)
	exit()

k = 0
for i in range(20):
    k += ans[i]*2**i
    
for i in range(n):
    b = B[i]
    if (i+1) & k != b:
        print(-1)
        exit()

print(k)