import collections,sys,math,functools,operator,itertools,bisect,heapq,decimal,string,time,random
n,k = map(int,input().split())
a = list(map(int,input().split()))
a.sort()
ans = 0
k-=1
cnt = n*(n-1)//2 - k
def calc(x,i):
    z = 0
    for j in a:
        l = (((j ^ x)>>i) & ((1 << 30)-1)) << i
        r = l + (1 << i)

        p = bisect.bisect_left(a,l)
        q = bisect.bisect_left(a,r)
        z += q-p
        #print(x,i,j,l,r,p,q)
        if l <= j < r:
            z -= 1
    z//=2
    #print(x,i,z)
   
    return z
res = 0
for i in reversed(range(30)):
    r = calc(ans+(1<<i),i)
    if res + r >= cnt:
        ans += 1<<i
        cnt -= res
        res = 0
    else:
        res += r

print(ans)