n,k=map(int,input().split())
k*=2
k+=n-1
a=list(map(int,input().split()))

now=[[a,a]]

ans=0
cnt=0

for i in range(30)[::-1]:
  xx=[]
  yy=[]
  c=0
  p=1<<i
  for x,y in now:
    x0=0
    x1=0
    y0=0
    y1=0
    for v in x:
      if v&p:
        x1+=1
      else:
        x0+=1
    for v in y:
      if v&p:
        y1+=1
      else:
        y0+=1
    c+=x0*y0+x1*y1
  now2=[]
  if cnt+c>k:
    for x,y in now:
      p1=[[],[]]
      p2=[[],[]]
      for v in x:
        if v&p:
          p1[0].append(v)
        else:
          p2[0].append(v)
      for v in y:
        if v&p:
          p1[1].append(v)
        else:
          p2[1].append(v)
      if p1[0] and p1[1]:
        now2.append(p1)
      if p2[0] and p2[1]:
        now2.append(p2)
        
  else:
    cnt+=c
    for x,y in now:
      p1=[[],[]]
      p2=[[],[]]
      for v in x:
        if v&p:
          p1[0].append(v)
        else:
          p2[1].append(v)
      for v in y:
        if v&p:
          p2[0].append(v)
        else:
          p1[1].append(v)
      if p1[0] and p1[1]:
        now2.append(p1)
      if p2[0] and p2[1]:
        now2.append(p2)
  now=now2
    
print(now[0][0][0]^now[0][1][0])