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

C = [dict() for _ in range(30)]
for i in range(N):
    for j in range(30):
        key = A[i] & ~((1<<(30-j))-1)
        bit = 1&A[i]>>(29-j)
        if (key, bit) in C[j]:
            C[j][(key, bit)] += 1
        else:
            C[j][(key, bit)] = 1

ans = 0
cnt = 0
flag = False
for i in range(30):
    c = 0
    for j in range(N):
        key = (ans^A[j]) & ~((1<<(30-i))-1)
        bit = 1&A[j]>>(29-i)
        if (key, bit) in C[i]:
            c += C[i][(key, bit)]
        if ans == 0:
            c -= 1
    c //= 2
    if cnt+c < K:
        ans |= 1<<(29-i)
        cnt += c

print(ans)