from collections import Counter

N,K=map(int,input().split())
A=list(map(int,input().split()))
K-=1
S=0

for i in range(30,-1,-1):
    d=Counter()

    for e in A:
        d[(e>>i)<<i]+=1
    
    cnt=0

    for key,val in d.items():
        if S==0:
            cnt+=val*(val-1)//2
        elif key^S in d and key<(key^S):
            cnt+=val*d[key^S]
    
    if cnt<=K:
        K-=cnt
        S|=1<<i

print(S)