n, k = input().split(" ")
n, k = int(n), int(k)

x = [int(i) for i in input().split(" ")]

def all_and_ret(musicians):
    ret = ~0
    for musician in musicians:
        ret &= musician
    return ret


def partition(musicians, index):
    partition_list = []
    for i in range(30):
        partition_list.append([])
    for musician in musicians:
        mask = 1
        for i in range(0, index):
            if musician & mask:
                partition_list[i].append(musician)
            mask <<= 1

    for i in range(len(partition_list) - 1, -1, -1):
        if len(partition_list[i]) == k:
            return all_and_ret(partition_list[i])
        elif len(partition_list[i]) > k:
            retval = partition(partition_list[i], i)
            if retval >= 0:
                return retval

    print("returning -1 :(")
    return -1

ret = partition(x, 30)

print(ret)