//marico el que lo lea
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <stdlib.h>
#include <assert.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> ii;

void fastIO() {std::ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);}

#define pb push_back
#define all(obj) obj.begin(), obj.end()
#define ms(obj, val) memset(obj, val, sizeof(obj))
#define ms2(obj, val, sz) memset(obj, val, sizeof(obj[0])*sz)

#define fst first
#define snd second

template<typename T, typename U> inline void mnze(T &x, U y) {if(y < x) x = y;}
template<typename T, typename U> inline void mxze(T &x, U y) {if(x < y) x = y;}

void _scan( int &x ) { scanf("%d",&x); }
void _scan( long long &x ) { scanf("%lld",&x); }
void _scan( double &x ) { scanf("%lf",&x); }
void _scan( char &x ) { scanf(" %c",&x); }
void _scan( char *x ) { scanf("%s",x); }
template<typename T>
void _scan( vector<T> &v ){
	for(int i=0; i<(int)v.size(); i++) _scan(v[i]);
}
void scan() {}
template<typename T, typename... U>
void scan( T& head, U&... tail ) { _scan(head); scan(tail...);}

template<typename T> void _dbg(const char* sdbg, T h)
	{ cerr<<sdbg<<"="<<h<<"\n"; }
template<typename T, typename... U> void _dbg(const char* sdbg, T h, U... t) {
	while(*sdbg != ',')cerr<<*sdbg++;
	cerr<<"="<<h<<","; _dbg(sdbg+1, t...);
}

#ifdef LOCAL
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
#define debugv(x) {{cerr<<#x<<" = "; for(auto _i:x) cerr<<_i<<", ";cerr<<"\n";}}
#define debuga(x, sz) {{cerr<<#x<<" = ";for(int _i=0;_i<sz;_i++)cerr<<x[_i]<<", ";cerr<<"\n";}}
#else
#define debug(...) (__VA_ARGS__)
#define debugv(x)
#define debuga(x, sz)
#define cerr if(0)cout
#endif

ll MOD = 1e9+7;

inline ll Msum(ll x){return x;}
template<typename... Rest>
inline ll Msum(ll x, Rest... rest){ return (x + Msum(rest...)) % MOD; }

inline ll Mprod(ll x){return x;}
template<typename... Rest>
inline ll Mprod(ll x, Rest... rest){ return x * Mprod(rest...) % MOD; }

inline ll Mnorm(ll x){ return (x%MOD + MOD) %MOD; }

inline ll Msq(ll x){ return (x*x) %MOD; }
inline ll Mpot(ll x, ll e){
	if(!e) return 1;
	return Msq(Mpot(x, e>>1)) * (e & 1 ? x : 1)%MOD;
}
////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

const int MAXN = 3e5+5;

int n, k;
char s[MAXN];
char ans[MAXN];

int main(){
	scan(n, k, s);

	ll pot=1;
	for(int i=0; i<k-1; i++){
		pot *= 2;
		if(pot >= n) break;
	}

	char mn = s[0];
	for(int i=0; i<n; i++) mnze(mn, s[i]);

	int cnt=0, len=0;
	for(int i=0; i<n; i++){
		if(s[i] == mn) cnt++;
		else cnt=0;
		mxze(len, cnt);
	}

	if(pot*len >= n){
		for(int i=0; i<n; i++) ans[i] = mn;
		printf("%s\n", ans);
		return 0;
	}

	for(int i=0; i<pot*len; i++) ans[i] = mn;
	for(int i=pot*len; i<n; i++) ans[i] = 'z';

	cnt = 0;
	for(int i=0; i<n; i++){
		if(s[i] == mn) cnt++;
		else cnt=0;

		if(i == n-1 && pot*2*cnt >= pot*len){
			if(pot*2*cnt >= n){
				for(int k=0; k<n; k++) ans[k] = mn;
			}else{
				bool lss=false;
				for(int k=0; k<(pot*2*cnt) - cnt; k++){
					if(mn < ans[k]) lss=true;
					ans[k] = mn;
				}
				int j=n-1;
				for(int k=(pot*2*cnt) - cnt; k<n; k++){
					if(lss){
						ans[k] = s[j];
					}else{
						if(s[j] < ans[k]){
							lss=true;
							ans[k] = s[j];
						}else if(s[j] > ans[k]) break;
					}
					j--;
				}
			}
		}else if(cnt == len){
			int dir=1;
			int j=i+1;
			bool lss = false;
			for(int k=pot*len; k<n; k++){
				if(j==n){
					j=n-1;
					dir=-1;
				}
				if(lss){
					ans[k] = s[j];
				}else{
					if(s[j] < ans[k]){
						lss=true;
						ans[k] = s[j];
					}else if(s[j] > ans[k]){
						break;
					}
				}
				j = j+dir;
			}
		}
	}
	printf("%s\n", ans);

}


Measured memory_bytes: 256