use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let mut iter = input.trim().split_whitespace();

    let n: usize = iter.next().unwrap().parse().unwrap();
    let k: usize = iter.next().unwrap().parse().unwrap();

    input.clear();
    io::stdin().read_line(&mut input).unwrap();
    let a: Vec<i32> = input
        .trim()
        .split_whitespace()
        .map(|x| x.parse().unwrap())
        .collect();

    let r = n * (n - 1) / 2 - k + 1;

    let mut next_node_id0 = vec![-1];
    let mut next_node_id1 = vec![-1];
    let mut parent_id = vec![-1];
    let mut depth = vec![0];
    let mut count = vec![0];
    let mut last = Vec::new();

    let mut nodes_id = 1;
    for &num in &a {
        let mut now = 0;
        for i in (0..=30).rev() {
            let next = if num & (1 << i) == 0 { 0 } else { 1 };

            if next == 0 {
                if next_node_id0[now] == -1 {
                    next_node_id0[now] = nodes_id as i32;
                    next_node_id0.push(-1);
                    next_node_id1.push(-1);
                    parent_id.push(now as i32);
                    depth.push(depth[now] + 1);
                    count.push(0);
                    now = nodes_id;
                    nodes_id += 1;
                } else {
                    now = next_node_id0[now] as usize;
                }
            } else {
                if next_node_id1[now] == -1 {
                    next_node_id1[now] = nodes_id as i32;
                    next_node_id0.push(-1);
                    next_node_id1.push(-1);
                    parent_id.push(now as i32);
                    depth.push(depth[now] + 1);
                    count.push(0);
                    now = nodes_id;
                    nodes_id += 1;
                } else {
                    now = next_node_id1[now] as usize;
                }
            }
        }
        count[now] += 1;
        last.push(now);
    }

    let mut count2 = vec![0; count.len()];
    for &x in &last {
        let mut node = x as i32;
        while node != -1 {
            count2[node as usize] += 1;
            node = parent_id[node as usize];
        }
    }

    let mut ok = 0;
    let mut ng = *a.iter().max().unwrap() + 1;

    while ng > ok + 1 {
        let mid = (ok + ng) / 2;
        let mut total_count = 0;

        for &num in &a {
            if total_count / 2 >= r {
                break;
            }
            let mut now = 0;
            let mut current_count = 0;
            let mut flag = false;

            for dep in (0..=30).rev() {
                let x = next_node_id0[now];
                let y = next_node_id1[now];
                let num_bit = num & (1 << dep);
                let mid_bit = mid & (1 << dep);

                match (num_bit == 0, mid_bit == 0) {
                    (true, true) => {
                        if y != -1 {
                            current_count += count2[y as usize];
                        }
                        if x != -1 {
                            now = x as usize;
                        } else {
                            flag = true;
                            break;
                        }
                    }
                    (true, false) => {
                        if y != -1 {
                            now = y as usize;
                        } else {
                            flag = true;
                            break;
                        }
                    }
                    (false, true) => {
                        if x != -1 {
                            current_count += count2[x as usize];
                        }
                        if y != -1 {
                            now = y as usize;
                        } else {
                            flag = true;
                            break;
                        }
                    }
                    (false, false) => {
                        if x != -1 {
                            now = x as usize;
                        } else {
                            flag = true;
                            break;
                        }
                    }
                }
            }

            if !flag {
                current_count += count2[now];
            }
            total_count += current_count;
        }

        if total_count / 2 >= r {
            ok = mid;
        } else {
            ng = mid;
        }
    }

    println!("{}", ok);
}