n = int(input())
a = [0] + list(map(int, input().split()))  #1スタート！

for i in range(1, n + 1):
    if a[i] & ~i:  
        print(-1)
        exit()

x = 0
for k in range(n.bit_length()):
    m = 1 << k 
    v = None  
    for i in range(1, n + 1):
        if i & m:
            b = 1 if (a[i] & m) else 0
            if v is None:
                v = b
            elif v != b:
                print(-1)
                exit()
    if v == 1:
        x |= m

if x == 0:
    x = 1 << n.bit_length()

print(x)