#include<bits/stdc++.h>
const int N=5010;
int n,k,l[N*2];
char s[N*2],mc='z';
int cmp(int i,int j){
	for (;s[i]==s[j];i++,j++);
	return s[i]<s[j];
}
int main(){
	scanf("%d%d%s",&n,&k,s);
	for (int i=0;i<n;i++)(s[n*2-i-1]=s[i])<mc?mc=s[i]:1;
	if (k>15)k=15;
	int ml=0;
	for (int i=0;i<n*2;i++){
		l[i+1]=s[i]==mc?l[i]+1:0;
		l[i+1]>ml?ml=l[i+1]:1;
	}
	int r=n-(ml<<k-1);
	if (r<0)r=0;
	int p=-1;
	for (int i=n;i<=n*2;i++)
		if (l[i]==ml&&(p<0||cmp(n*2-i+ml,p)))p=n*2-i+ml;
	for (int i=n-r;i--;)putchar(mc);
	for (int i=0;i<r;i++)putchar(s[p+i]);
	puts("");
}