class Trie:
    def __init__(self, N):
        self.tree = [[-1]*2 for _ in range(N+1)]
        self.arrive = [0]*(N+1)
        self.nex = 1
    
    def insert(self, S):
        now = 0
        for i in range(30):
            b = 1&S>>(29-i)
            if self.tree[now][b] == -1:
                self.tree[now][b] = self.nex
                now = self.nex
                self.nex += 1
            else:
                now = self.tree[now][b]
            self.arrive[now] += 1
    
    def query(self, MAX, S):
        now = 0
        ans = 0
        xor = MAX^S
        for i in range(30):
            b = 1 & S>>(29-i)
            if 1<<(29-i) & MAX:
                if b == 0 and self.tree[now][0] != -1:
                    ans += self.arrive[self.tree[now][0]]
                elif b == 1 and self.tree[now][1] != -1:
                    ans += self.arrive[self.tree[now][1]]
            b2 = 1 & xor>>(29-i)
            if self.tree[now][b2] != -1:
                now = self.tree[now][b2]
            else:
                break
        return ans

N, K = map(int, input().split())
A = list(map(int, input().split()))

def func(n):
    T = Trie(N*30)
    ans = 0
    for i in reversed(range(N)):
        T.insert(A[i])
        ans += T.query(n, A[i])-1
    del T
    return ans

left = 0
right = (1<<30)-1
while left+1 < right:
    mid = (left+right)//2
    if func(mid) < K:
        left = mid
    else:
        right = mid

print(right-1)