using System;
using System.Collections.Generic;
using System.Linq;
class Program
{
static string InputPattern = "InputX";
static List<string> GetInputList()
{
var WillReturn = new List<string>();
if (InputPattern == "Input1") {
WillReturn.Add("7 3");
WillReturn.Add("atcoder");
//acd
}
else if (InputPattern == "Input2") {
WillReturn.Add("14 5");
WillReturn.Add("kittyonyourlap");
//inlap
}
else {
string wkStr;
while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
}
return WillReturn;
}
static void Main()
{
List<string> InputList = GetInputList();
int[] wkArr = InputList[0].Split(' ').Select(pX => int.Parse(pX)).ToArray();
int K = wkArr[1];
string S = InputList[1];
// 添え字のList[文字]なDict
var IndListDict = new Dictionary<char, List<int>>();
int UB = S.Length - 1;
for (int I = 0; I <= UB; I++) {
char CurrChar = S[I];
if (IndListDict.ContainsKey(CurrChar) == false) {
IndListDict[CurrChar] = new List<int>();
}
IndListDict[CurrChar].Add(I);
}
var InsSparseTable = new SparseTable<char>(S, true);
int CurrInd = 0;
var sb = new System.Text.StringBuilder();
while (sb.Length < K) {
int NeedLen = K - sb.Length;
int RangeMax = UB - NeedLen + 1;
char MinChar = InsSparseTable.Query(CurrInd, RangeMax);
sb.Append(MinChar);
int ResultInd = ExecNibunhou_LowerBound(CurrInd, IndListDict[MinChar]);
CurrInd = IndListDict[MinChar][ResultInd] + 1;
}
Console.WriteLine(sb.ToString());
}
// 二分法で、Val以上で最小の値を持つ、添字を返す
static int ExecNibunhou_LowerBound(int pVal, List<int> pList)
{
// 最後の要素がVal未満の特殊ケース
if (pVal > pList.Last()) {
return -1;
}
// 最初の要素がVal以上の特殊ケース
if (pVal <= pList[0]) {
return 0;
}
int L = 0;
int R = pList.Count - 1;
while (L + 1 < R) {
int Mid = (L + R) / 2;
if (pList[Mid] >= pVal) {
R = Mid;
}
else {
L = Mid;
}
}
return R;
}
}
#region SparseTable
internal class SparseTable<Type> where Type : IComparable<Type>
{
// 最小値か最大値[2の指数][開始Ind]なArr
private Type[][] mMinOrMaxJagArr;
// Log2の値[2べき値] なDict
private Dictionary<long, long> mLogDict = new Dictionary<long, long>();
// 対応する2べき値[クエリの区間長]
private long[] mBeki2Arr;
// 最小値を取得するか？
private bool mIsMin = false;
// コンストラクタ1(配列が引数)
internal SparseTable(Type[] pInitArr, bool pIsMin)
{
mIsMin = pIsMin;
long Sisuu = 0;
long Beki2 = 1;
while (true) {
mLogDict[Beki2] = Sisuu;
Beki2 *= 2;
if (Beki2 > pInitArr.Length) {
break;
}
Sisuu++;
}
mMinOrMaxJagArr = new Type[Sisuu + 1][];
foreach (var EachPair in mLogDict) {
long CurrBeki2 = EachPair.Key;
long CurrSisuu = EachPair.Value;
long CurrUB = pInitArr.GetUpperBound(0) - CurrBeki2 + 1;
mMinOrMaxJagArr[CurrSisuu] = new Type[CurrUB + 1];
}
// 長さ1の分を初期化
for (long I = 0; I <= pInitArr.GetUpperBound(0); I++) {
mMinOrMaxJagArr[0][I] = pInitArr[I];
}
// コンストラクタのヘルパメソッド
ConstructorHelper();
// 対応する2べき値[クエリの区間長]を設定
SetBeki2Arr(pInitArr.Length);
}
// コンストラクタ2(Listが引数)
internal SparseTable(List<Type> pInitList, bool pIsMin)
{
mIsMin = pIsMin;
long Sisuu = 0;
long Beki2 = 1;
while (true) {
mLogDict[Beki2] = Sisuu;
Beki2 *= 2;
if (Beki2 > pInitList.Count) {
break;
}
Sisuu++;
}
mMinOrMaxJagArr = new Type[Sisuu + 1][];
foreach (var EachPair in mLogDict) {
long CurrBeki2 = EachPair.Key;
long CurrSisuu = EachPair.Value;
long CurrUB = pInitList.Count - 1 - CurrBeki2 + 1;
mMinOrMaxJagArr[CurrSisuu] = new Type[CurrUB + 1];
}
// 長さ1の分を初期化
for (int I = 0; I <= pInitList.Count - 1; I++) {
mMinOrMaxJagArr[0][I] = pInitList[I];
}
// コンストラクタのヘルパメソッド
ConstructorHelper();
// 対応する2べき値[クエリの区間長]を設定
SetBeki2Arr(pInitList.Count);
}
// コンストラクタ3(列挙が引数)
internal SparseTable(IEnumerable<Type> pInitEnum, bool pIsMin)
{
mIsMin = pIsMin;
Type[] InitArr = pInitEnum.ToArray();
long Sisuu = 0;
long Beki2 = 1;
while (true) {
mLogDict[Beki2] = Sisuu;
Beki2 *= 2;
if (Beki2 > InitArr.Length) {
break;
}
Sisuu++;
}
mMinOrMaxJagArr = new Type[Sisuu + 1][];
foreach (var EachPair in mLogDict) {
long CurrBeki2 = EachPair.Key;
long CurrSisuu = EachPair.Value;
long CurrUB = InitArr.GetUpperBound(0) - CurrBeki2 + 1;
mMinOrMaxJagArr[CurrSisuu] = new Type[CurrUB + 1];
}
// 長さ1の分を初期化
for (int I = 0; I <= InitArr.GetUpperBound(0); I++) {
mMinOrMaxJagArr[0][I] = InitArr[I];
}
// コンストラクタのヘルパメソッド
ConstructorHelper();
// 対応する2べき値[クエリの区間長]を設定
SetBeki2Arr(InitArr.Length);
}
// 対応する2べき値[クエリの区間長]を設定
private void SetBeki2Arr(long pDataLen)
{
long[] Beki2Arr = mLogDict.Keys.ToArray();
mBeki2Arr = new long[pDataLen + 1];
long CurrBeki2 = 1;
for (long I = 0; I <= pDataLen; I++) {
mBeki2Arr[I] = CurrBeki2;
if (mLogDict.ContainsKey(I)) {
CurrBeki2 = I;
}
}
}
// コンストラクタのヘルパメソッド
private void ConstructorHelper()
{
for (long LoopLength = 2; LoopLength < long.MaxValue; LoopLength *= 2) {
if (mLogDict.ContainsKey(LoopLength) == false) break;
long HalfRange = LoopLength / 2;
Type[] CurrArr = mMinOrMaxJagArr[mLogDict[HalfRange]];
Type[] SendArr = mMinOrMaxJagArr[mLogDict[LoopLength]];
for (long I = 0; I <= SendArr.GetUpperBound(0); I++) {
long StaInd1 = I;
long EndInd1 = I + HalfRange - 1;
long StaInd2 = EndInd1 + 1;
long EndInd2 = StaInd2 + HalfRange - 1;
Type Val1 = CurrArr[I];
Type Val2 = CurrArr[StaInd2];
int CompResult = Val1.CompareTo(Val2);
if (mIsMin) {
SendArr[I] = (CompResult >= 1 ? Val2 : Val1);
}
else {
SendArr[I] = (CompResult >= 1 ? Val1 : Val2);
}
}
}
}
// 閉区間 [L,R]の最小値または最大値を求める
internal Type Query(long pL, long pR)
{
// 区間を内包する2べき値を使用
long RangeLen = pR - pL + 1;
long Beki2 = mBeki2Arr[RangeLen];
long Sisuu = mLogDict[Beki2];
Type Val1 = mMinOrMaxJagArr[Sisuu][pL];
Type Val2 = mMinOrMaxJagArr[Sisuu][pR - Beki2 + 1];
int CompResult = Val1.CompareTo(Val2);
if (mIsMin) {
return (CompResult >= 1 ? Val2 : Val1);
}
return (CompResult >= 1 ? Val1 : Val2);
}
}
#endregion