from bisect import bisect_left, bisect_right

N, K = map(int, input().split())
A = list(map(int, input().split()))

A.sort()


# A の中で l 以上 r 以下の要素の個数
def count(l, r):
    return bisect_right(A, r) - bisect_left(A, l)


ans = 0
X = 2 * K + N

for i in range(30, -1, -1):
    left = ans
    right = ans | ((1 << i) - 1)

    cnt = 0

    for a in A:
        l = left ^ (a & ~((1 << i) - 1))
        r = right ^ (a & ~((1 << i) - 1))
        cnt += count(l, r)

    if cnt < X:
        ans |= 1 << i
        X -= cnt

print(ans)