N = int(input())
B = list(map(int, input().split()))

digit = [None] * 20


def check(shift, num):
    if digit[shift] is None:
        digit[shift] = num
    elif digit[shift] != num:
        return False
    return True


for i, b in enumerate(B, start=1):
    for j in range(20):
        match ((i >> j) & 1, (b >> j) & 1):
            case (1, 0):
                if not check(j, 0):
                    print(-1)
                    exit()
            case (0, 1):
                print(-1)
                exit()
            case (1, 1):
                if not check(j, 1):
                    print(-1)
                    exit()
            case _:
                pass

ans = sum([(0 if digit[i] is None else digit[i]) << i for i in range(20)])
print(ans)