using Lib;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using static Lib.OutputLib;
public class Solver
{
    const bool MultiTestCase = false;
    void Solve()
    {
        int n = ri;
        long k = rl;
        ReadArray(out int[] a, n);

        var trie = new IntegerTrie();
        for (int i = 0; i < n; i++) trie.Add(a[i]);
        var dic = new Dictionary<long, long>();
        for (int i = 0; i < n; i++) if (!dic.ContainsKey(a[i])) dic.Add(a[i], 1); else dic[a[i]]++;

        k = k * 2 + n;
        Write(trie.MultiXORGet(k - 1, dic.Select(p => (p.Key, p.Value)).ToArray()));
    }

#pragma warning disable CS0162
    public Solver() { if (!MultiTestCase) Solve(); else for (int t = ri; t > 0; t--) Solve(); }
#pragma warning restore CS0162

    const int IINF = 1 << 30;
    const long INF = 1L << 60;
    int ri { [MethodImpl(256)] get => (int)sc.Integer(); }
    long rl { [MethodImpl(256)] get => sc.Integer(); }
    uint rui { [MethodImpl(256)] get => (uint)sc.UInteger(); }
    ulong rul { [MethodImpl(256)] get => sc.UInteger(); }
    double rd { [MethodImpl(256)] get => sc.Double(); }
    string rs { [MethodImpl(256)] get => sc.Scan(); }
    string rline { [MethodImpl(256)] get => sc.Line(); }
    public StreamScanner sc = new StreamScanner(Console.OpenStandardInput());
    void ReadArray(out int[] a, int n) { a = new int[n]; for (int i = 0; i < a.Length; i++) a[i] = ri; }
    void ReadArray(out long[] a, int n) { a = new long[n]; for (int i = 0; i < a.Length; i++) a[i] = rl; }
    void ReadArray<T>(out T[] a, int n, Func<T> read) { a = new T[n]; for (int i = 0; i < a.Length; i++) a[i] = read(); }
    void ReadArray<T>(out T[] a, int n, Func<int, T> read) { a = new T[n]; for (int i = 0; i < a.Length; i++) a[i] = read(i); }
}

static class Program
{
    static public void Main(string[] args)
    {
        SourceExpander.Expander.Expand();
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver();
        Console.Out.Flush();
    }
}
#region Expanded by https://github.com/kzrnm/SourceExpander
namespace Lib{public class IntegerTrie{public const int B=62;class Node{public int Count;public Node?[]Next;public Node(){Count=0;Next=new Node?[]{null,null};}}Node root;public IntegerTrie(){root=new Node();}public int Size=>root.Count;[MethodImpl(256)]public void Add(long v,int count=1){Node x=root;for(int b=B-1;b>=0;b--){x.Count+=count;var f=(v>>b)&1;if(x.Next[f]==null)x.Next[f]=new Node();x=x.Next[f]!;}x.Count+=count;}[MethodImpl(256)]public void Remove(long v){Node x=root;for(int b=B-1;b>=0;b--){x.Count--;x=x.Next[(v>>b)&1]!;}x.Count--;}[MethodImpl(256)]public long Max(long k=0)=>Min(~k);[MethodImpl(256)]public long Min(long k=0){Node x=root;long ret=0;for(int b=B-1;b>=0;b--){long f=(k>>b)&1,r=0;if(x.Next[f]==null||x.Next[f]!.Count==0)r=1;if(r==1)ret|=1L<<b;x=x.Next[f^r]!;}return ret;}[MethodImpl(256)]public long LowerBound(long v,long k=0)=>CountLower(v,k);[MethodImpl(256)]public long UpperBound(long v,long k=0)=>CountLower(v+1,k);[MethodImpl(256)]public int Count()=>root.Count;[MethodImpl(256)]public int Count(long v){Node?x=root;for(int i=B-1;i>=0;i--){x=x.Next[(v>>i)&1];if(x==null)return 0;}return x.Count;}[MethodImpl(256)]public long Get(int i,long k=0){Node x=root;long ret=0;for(int b=B-1;b>=0;b--){long f=(k>>b)&1;int m=x.Next[f]==null?0:x.Next[f]!.Count;if(i<m){x=x.Next[f]!;}else{i-=m;x=x.Next[f^1]!;ret|=1L<<b;}}return ret;}[MethodImpl(256)]private long CountLower(long v,long k){Node?x=root;int ret=0;for(int b=B-1;b>=0;b--){long f=(k>>b)&1,r=(v>>b)&1;if(r==1&&x.Next[f]!=null)ret+=x.Next[f]!.Count;x=x.Next[f^r];if(x==null)break;}return ret;}public long MultiXORGet(long p,ReadOnlySpan<(long val,long cnt)>ks){int n=ks.Length;var xs=new Node?[n];for(int i=0;i<n;i++)xs[i]=root;long ret=0,count=0;for(int b=B-1;b>=0;b--){long c=0;for(int i=0;i<n;i++){if(xs[i]==null)continue;var(val,cnt)=ks[i];int k=(int)((val>>b)&1);if(xs[i]!.Next[k]!=null)c+=xs[i]!.Next[k]!.Count*cnt;}if(count+c<=p){ret|=1L<<b;count+=c;for(int i=0;i<n;i++){if(xs[i]==null)continue;int k=(int)((ks[i].val>>b)&1)^1;xs[i]=xs[i]!.Next[k];}}else{for(int i=0;i<n;i++){if(xs[i]==null)continue;int k=(int)((ks[i].val>>b)&1);xs[i]=xs[i]!.Next[k];}}}return ret;}}}
namespace Lib{public partial class StreamScanner{public StreamScanner(Stream stream){str=stream;}private readonly Stream str;private readonly byte[]buf=new byte[1024];private int len,ptr;public bool isEof=false;public bool IsEndOfStream{get{return isEof;}}[MethodImpl(256)]private byte Read(){if(isEof)throw new EndOfStreamException();if(ptr>=len){ptr=0;if((len=str.Read(buf,0,1024))<=0){isEof=true;return 0;}}return buf[ptr++];}[MethodImpl(256)]public char Char(){byte b;do b=Read();while(b<33||126<b);return(char)b;}[MethodImpl(256)]public string Line(){var sb=new StringBuilder();for(var b=Char();b!=10&&!isEof;b=(char)Read())sb.Append(b);return sb.ToString();}[MethodImpl(256)]public string Scan(){var sb=new StringBuilder();for(var b=Char();b>=33&&b<=126;b=(char)Read())sb.Append(b);return sb.ToString();}[MethodImpl(256)]public long Integer(){long ret=0;var ng=false;byte b;do b=Read();while(b!='-'&&(b<'0'||'9'<b));if(b=='-'){ng=true;b=Read();}for(;'0'<=b&&b<='9';b=Read())ret=ret*10+(b^'0');return ng?-ret:ret;}[MethodImpl(256)]public ulong UInteger(){ulong ret=0;byte b;do b=Read();while(b<'0'||'9'<b);for(;'0'<=b&&b<='9';b=Read())ret=ret*10+(ulong)(b^'0');return ret;}[MethodImpl(256)]public double Double()=>double.Parse(Scan());}}
namespace Lib{public static class OutputLib{[MethodImpl(256)]public static void WriteJoin<T>(string s,IEnumerable<T>t)=>Console.WriteLine(string.Join(s,t));[MethodImpl(256)]public static void WriteMat<T>(T[,]a,string sep=" "){int sz1=a.GetLength(0),sz2=a.GetLength(1);var b=new T[sz2];for(int i=0;i<sz1;i++){for(int j=0;j<sz2;j++)b[j]=a[i,j];WriteJoin(sep,b);}}[MethodImpl(256)]public static void WriteMat<T>(T[][]a,string sep=" "){foreach(var ar in a)WriteJoin(sep,ar);}[MethodImpl(256)]public static void WriteMat<T>(T[][]a,Func<T,string>map,string sep=" "){foreach(var ar in a)WriteJoin(sep,ar.Select(x=>map(x)));}[MethodImpl(256)]public static void Write(object t)=>Console.WriteLine(t.ToString());[MethodImpl(256)]public static void Write(params object[]arg)=>Console.WriteLine(string.Join(" ",arg.Select(x=>x.ToString())));[MethodImpl(256)]public static void Write(string str)=>Console.WriteLine(str);[MethodImpl(256)]public static void WriteFlush(object t){Console.WriteLine(t.ToString());Console.Out.Flush();}[MethodImpl(256)]public static void WriteError(object t)=>Console.Error.WriteLine(t.ToString());[MethodImpl(256)]public static void Flush()=>Console.Out.Flush();[MethodImpl(256)]public static void YN(bool t)=>Console.WriteLine(t?"YES":"NO");[MethodImpl(256)]public static void Yn(bool t)=>Console.WriteLine(t?"Yes":"No");[MethodImpl(256)]public static void yn(bool t)=>Console.WriteLine(t?"yes":"no");[MethodImpl(256)]public static void DeleteLine()=>Console.Write("\x1b[1A\x1b[2K");[MethodImpl(256)]public static void ProgressBar(long now,long total,int blocks=50){int x=(int)((2*now*blocks+1)/(2*total));Console.Write($"\x1b[G[\x1b[42m{string.Concat(Enumerable.Repeat("_",x))}\x1b[0m{string.Concat(Enumerable.Repeat("_",blocks-x))}] : {now} / {total}");}}}
namespace SourceExpander{public class Expander{[Conditional("EXP")]public static void Expand(string inputFilePath=null,string outputFilePath=null,bool ignoreAnyError=true){}public static string ExpandString(string inputFilePath=null,bool ignoreAnyError=true){return "";}}}
#endregion Expanded by https://github.com/kzrnm/SourceExpander