from collections import defaultdict
N,K=map(int,input().split())
K-=1
A=list(map(int,input().split()))
exp2=[2**i for i in range(35)]
left_sum=0
left=0
ans=0
for i in range(29,-1,-1):
    left+=exp2[i]
    dic=defaultdict(int)
    for j in range(N):
        dic[A[j]&(left)]+=1
    tmp_sum=left_sum
    keys = list(dic.keys())
    for k in keys:
        if k==k^ans:
            tmp_sum+=dic[k]*(dic[k^ans]-1)
        else:
            tmp_sum+=dic[k]*(dic[k^ans])
    if tmp_sum<=K*2:
        ans+=exp2[i]
        left_sum=tmp_sum
print(ans)