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:
    now_xx=[[],[]]
    now_yy=[[],[]]
    for v in x:
      now_xx[bool(v&p)].append(v)
    for v in y:
      now_yy[bool(v&p)].append(v)
    xx.append(now_xx)
    yy.append(now_yy)
    c+=len(now_xx[0])*len(now_yy[0])+len(now_xx[1])*len(now_yy[1])
  now=[]
  if cnt+c>k:
    for v in range(len(xx)):
      if xx[v][0] and yy[v][0]:
        now.append([xx[v][0],yy[v][0]])
      if xx[v][1] and yy[v][1]:
        now.append([xx[v][1],yy[v][1]])
  else:
    cnt+=c
    for v in range(len(xx)):
      if xx[v][0] and yy[v][1]:
        now.append([xx[v][0],yy[v][1]])
      if xx[v][1] and yy[v][0]:
        now.append([xx[v][1],yy[v][0]])
    
print(now[0][0][0]^now[0][1][0])