N = int(input())
B = list(map(int, input().split()))
x = 0
for i, b in enumerate(B, 1):
    for j in range(30):
        if b >> j & 1:
            x |= 1 << j
for i, b in enumerate(B, 1):
    if i & x != b:
        print(-1)
        exit()
print(x)